Specifically, Epsilon-iota (ει) initially became /eː/ in Classical Greek before it later raised to (ι) while, later, omicron-iota (οι) and upsilon-iota (υι) merged with upsilon (υ).
In Modern Greek, the letters and digraphs ι, η, υ, ει, οι, υι (rare) are all pronounced [i].
As an example of a relatively minor (almost insignificant) source of variant readings, some ancient manuscripts spelled words the way they sounded, such as the 4th-century Codex Sinaiticus, which sometimes substitutes a plain iota for the epsilon-iota digraph and sometimes does the reverse.
[1] English-speaking textual critics use the word "itacism" to refer to the phenomenon and extend it loosely for all inconsistencies of spelling involving vowels.
[3] Against the "Erasmian" theory came the German humanist Johannes Reuchlin (1455-1522), in whose honor the Byzantine Greek pronunciation is also called Reuchlinian.