What is the origin or basis of the Polish and Hungarian prevalence for pronouncing "oo" vowels as "ee"? The variants of other vowels are easier to understand in the context of phonological shifting, but this in particular seems especially different. I would be interested to hear of both linguistic explanations as well as any homiletic reasons given for the difference.
I feel like I have come across two bits of relevant data, although I can't remember the sources. On one hand I feel like I read somewhere that the different pronunciations are associated with Mishnaic pronunciation in Eretz Yisroel and Bavel, although I can't remember which is which. I also feel like the Gemara says that the sounds are interchangeable, but I could be wrong on that too.