Christ is Risen!
Maybe, God willing I can help a little, a least I hope I can. You ask what was the liturgy of the early church like, has it changed any substantial way, if so, why can't it continue changing--perhaps including technological "enhancements" of one sort or another.
Has the Church changed since's it's beginning? The answer has to be both yes and no. Does a seed of corn change to become a stalk? Yes and no. The form certainly changes. It begins as a smooth tooth shaped seed and soon enough has roots and long floppy leaves; it grows tassels and in the end is hundreds if not thousands of times larger than the original seed. Yet, seed and stalk are ontologically contiguous. They are the same life without addition or subtraction. The stalk was what the seed needed to be in order to make more seed…just like itself. There, by the way is one of the great tests of the faithfulness of the Church…is it capable and does it still reproduce in kind after caliber of those celebrated Christians of the first Pentecost. I would say given the evidence of such notables as St. Seraphim of Sarov, St. Silouan, Mother Gavrielle, and Elders Sophrony, and Paisios (among several others), the first seed is still growing strong.
So how has the Church changed without changing, especially with respect to the liturgy and how Christians worship? And, what drove such changes as have been made. Christian worship has two historical taproots, one in the worship belonging to the synagog, which itself is a ancient historical outgrowth of the main and most ancient taproot, the worship belonging to the Temple. That worship was transferred whole from the worship associated with the Tabernacle, and that worship was revealed to Moses on Mt. Sinai, and first implemented by the High Priest Aaron together with his sons. So our worship is not, nor ever can be ad hoc, or be revised/supplemented to suit some individual or private groups religio-creative impulses. Where it develops, where it changes, it is in a manner consistent with and expressive of its unchanging ontology. The liturgy is adapted to our need and it has adapted to meet our need a handful of times across the centuries.
The first liturgy is attributed to St. James, back in NT times, but from what we know of those ancient times, its implementation in the early apostolic era had features we are not so accustomed to, even though some are mentioned by St. Paul. For example, another poster mentioned the original meal associated with the Divine Liturgy. Believe it or not after the breaking of the bread portion of the Liturgy (the part we are most familiar with) tables were lined up down the center of the church, with the head table resting against the Holy Altar. The monastic table is still consider the mystical and historical extension of this practice, which is why there are some particular customs and observations that go with sharing a monastic meal. St. Paul mentions these feasts as sometimes being an occasion where the well to do leave out the less well to do when everyone brought food for the feast. Unfortunately, the stingy too often remained stingy, and those just looking to graze off other's labors grew too wanton at these feasts forcing the Bishops to little by little discontinue this practice until it was all but unheard of outside the monastic preservation by the 4th century. So we lost the meal almost entirely….but one little bit remains in every Orthodox parish…the gift of the antidoran after Holy Communion.
The Divine Liturgy itself continued wherever Christian communities rooted, and a number of variants grew up in those different communities around the Mediterranean. These liturgies go back to St. James' liturgy joined together with related material incorporated from founding other apostles and apostolic era witnesses. These are the earliest versions of the ancient liturgies such as the Qurbana, the Liturgy of St. Mark, the Baradak of the Armenians, and Sarum liturgy of the early Roman Church. As one poster noted, some of those early celebrations were long indeed, even up to 8 hours. Of course those communities who did such things, like the Egyptian Church in the time of St. Mark and his disciples, believed the light was the proper time for worship and hearing of scripture…eating, drinking, physical labor, bodily needs, etc. those all belonged to dark hours and were saved until then. I doubt many of us today are so able and so hearty as those worthy forbearers in the faith. Given the needs of the time service grew shorter for most Christians. This was especially so after its legalization and the rise of monastic communities. The monastics tried to preserve the more rigorous ancient practice as much as possible while the laity in the world, who had to deal with the cares of the world were given shorter (but still long by our standards) services. There was one major redaction under St. Basil who got it down to about 2 to 3 hours, depending, and a generation later St. John Chrysostom pared out some redundant sets of prayers (holdovers from longer fuller services that included associated psalms and hymns) which is what we still use today…which basically means the core form of the Divine Liturgy has been settled since the 5th century more or less.
That said, all was not said with respect to length. When the Slavs converted, they were converted by monastics and took the monastic worship forms a normative for everyone…so much so that for a couple of centuries at least Russians lived and worked like a great national monastery. Indeed it was joked in other Orthodox lands that the Russians must have feet of iron since their services were so long…often 6 to 8 hours.
During this time there was one other substantive change…not so much in the liturgy itself per se, but in how it was experienced. The first Christian temples, outside of home churches, were built on the model of the synagog. Women stood on one side, men on the other, and the bishop sat on his bema in the middle from whence he taught all. A railing or open screen separated the men's and women's side, broken at the bema. The altar stood near the eastern wall on the men's side and when it was time to commune the women crossed to the men's side and gathered around altar with them. In time this partition moved over to enclose the altar in the from of a railing. In the eastern church this railing grew up to become the iconostasis and in the western church it became the Rood Screen. This brings us to the other major change.
In the centuries when the Church was subjected to persecution in the Roman empire, all worship was not done at the same time in all places. Recall the double root of Christian worship. On one side…the teaching side, it adapted the uses of the synagog to the Christian faith. On the sacrificial side the Church adapted the uses of the Temple to the needs of the Divine Liturgy…indeed the Divine Liturgy is the direct continuation of Temple worship where the Holy Gifts replace the symbolic gifts of slaughtered bulls and goats. That worship was only for the Holy…those who were baptized, in good standing and ready to receive the Body and Blood of Christ. Everyone else was dismissed. Holy things were truly only for the holy and not anyone else. The deacon's cry of "the doors, the doors" was taken literally…all egress as locked down to ensure the solemnity and privacy of the Divine Liturgy. In fact, it was so private, the baptized kept its details pretty much to themselves. New Christians didn't learn it all until they emerged from their baptismal washing. This was common for much that touched upon the worship life of Christians. We learn from St. Basil in the 4th century that it was only in his day were certain of these things that had been passed down orally for centuries being committed to writing for two reasons…one to not risk loosing something precious to the memory of the Church given the advancing age and numbers in the Church, second the reasons for the heroic level of secrecy no longer existed. The faith was now legal. There was no longer the risk that authorities would sees upon details of the inner life and practice of the Church and hold it up to ridicule, as Rome did with what was more commonly known of the more external matters regarding the life of the Church.
It was during this time when the secrets were being written down that eastern screens became permanently covered with icons, where in the levant, India, and Western Europe a large curtain became a fixture. The curtain is found in Eastern Churches too…but it just covers the Holy Doors, not the entire altar area. This curtain, the later iconstasis were put permanently in place to address the needs that arose both from no longer "actually" dismissing the catechumens and penitents. Remember, Holy Things are for the Holy. By the 4th and 5th centuries there were many in Christian ranks, not all the Baptized lived up to the standards expected of the faithful…they were effectively baptized catechumens. It was not proper for them to witness the Divine Liturgy. It was for participants only, not spectators. All that was being achieved mystically in the altar was represented to the eyes of the people in the icons, without exposing the Holy Eucharist itself to profane curiosities.
The curtains and the icon screens serve to remind us just why such changes that have been made over time have been made. They've been made because of our sins, because we are less faithful than those who came before us, less able, weaker, and more in need of those condescensions to our collective infirmities than those of our ancient Christian forbearers. We struggle to keep morally clean, and fail more often than not at that, let alone do any mighty works for Christ's sake. Most of us cannot live as they lived and worship as robustly as they worship…it more likely destroy our faith than to nurture it if we tried off the bat as a community to live as they lived. God is merciful, and meets us where we are without compromising the life He offers us.
However, if one feels called and is willing to step up to the challenge of the Christian life at it's ancient levels of zeal and rigor, one need only enter the ranks of the monastics…and all that NT glory is there, day long services, simple lives, holiness, hyper-reverent communion, the forsaking of all things to follow Christ…and who knows, you might even get to know some of those modern day apostolic caliber seeds as they ripen in the monastic ear.
And there, I think you have it in modest outline.