Debates re. the biblical texts notwithstanding, that the Church (East and West -all regions) did not use unleavened bread for the Eucharist until the 9th century AD is the unanimous verdict of contemporary historical scholarship (see below). The only exceptions to this were the Ebionites (earliest example) and the Armenians who introduced unleavened bread in the 7th century. If I'm not mistaken, Alcuin, an 8th century scholar in the court of Charlemagne, provides the earliest undisputed reference to Eucharistic usage of unleavened bread in the Roman Catholic Church. Whether the NT presents a contradictory picture, or whether the data of the Synoptics vs. John may be harmonized to defend one or the other pictures therein is still disputed in contemporary scholarship.
A few excerpts, including footnotes, from Jonathan Klawans, “Was Jesus’ Last Supper a Sedar?” (Biblical Archaeology Review) http://www.bib-arch.org/e-features/jesus-last-supper.asp#note23r
"Contrary to popular belief, the Catholic custom of using unleavened wafers in the Mass is medieval in origin. The Orthodox churches preserve the earlier custom of using leavened bread.23 Is it not possible to see the switch from using leavened to unleavened bread as a “Passoverization” of sorts?"
23. On the medieval debate between the Catholic and Orthodox churches on this matter, see Jaroslav Pelikan, The Christian Tradition: A History of the Development of Doctrine, vol. 2, The Spirit of Eastern Christendom (600–1700)
(Chicago: Univ. of Chicago Press, 1971), pp. 177–178. On the archaeological evidence pertaining to this dispute, see George Galavaris, Bread and the Liturgy: The Symbolism of Early Christian and Byzantine Bread Stamps
(Madison: Univ. of Wisconsin Press, 1970).
“I want to operate here under the opposite assumptions: that the Gospels can tell us about the historical Jesus,3 and that rabbinic sources can be used—with caution—to reconstruct what Jews at the time of Jesus might have believed and practiced.4 Even so, I do not think the Last Supper was a Passover Seder.”
...A number of scholars now believe that the ritual context for the Last Supper was not a Seder but a standard Jewish meal. That Christians celebrated the Eucharist on a daily or weekly basis (see Acts 2:46–47) underscores the fact that it was not viewed exclusively in a Passover context (otherwise, it would have been performed, like the Passover meal, on an annual basis).
"An ancient Christian church manual called the Didache also suggests that the Last Supper may have been an ordinary Jewish meal. In Chapters 9 and 10 of the Didache, the eucharistic prayers are remarkably close to the Jewish Grace After Meals (Birkat ha-Mazon).