Hmm. It seems that nothing has changed in the written Greek except the accent marks, but I'd guess the pronunciation has changed significantly.
Yes, the pronunciation (just like grammar and syntax) is simplified, but we retain what is called historical orthography, that is the Attic-Ionic variant of the written classical Greek of 5th-4th century BC. And yes, the pronunciation has altered (a long process, started in the 4th century BC-->η lost its long ε pronunciation and began pronounced as plain iota, ι (ee) and continued until the 14th-15th century AD-->ει, οι, υ pronounced as plain ι (ee) also).
However, if I might add, the Greek spoken today is closer to Ancient Greek than the Greek spoken two-three hundred years ago, because of the massive influence of the ancient language in all areas, from vocabulary, to inflection, to syntax (especially after the movement for the "purification" of the language from its Turkish-Venetian influence, the Katharevousa
form of language, early 19th century).
Some Venetian or Turkish works are present though, in colloquial, informal language.
H Μπάκα (Baka, fem.)-->from the Venetian "Bacca", which although in Italian means "a fruit derived from a single ovary having one or many seeds within a fleshy wall or pericarp"-->grape, tomato, in colloquial Greek means the big belly, usually the beer-belly