Dear Alex,
This is all I know about the matter:
Pre-Nikonian Slavonic and Russian, used by Old Believers and Ruthenians alike: ICYC¯b (the last letter is an attempt to write a
tvërdyj znak). It is pronounced "eeSOOS'. (Or was it UCYC¯b? It would be pronounced the same.)
Post-Nikonian Slavonic and pre-1918 Russian, used by Russian Orthodox: IUCYC¯b, pronounced "eeyeeSOOS'. In old Russian spelling, I, i was used for ee/yee before a vowel.
Modern Russian: UUCYC, pronounced the same as the post-Nikonian and old Russian spelling.
I don't know the reason behind Patriarch Nikon's change.
http://oldworldrus.com [ 12-06-2001: Message edited by: Serge ]