> I just want to understand why the Turks decided to change this letter, and this letter only

Because Turkish uses a phonetic alphabet suited for Turkish sounds, based on latin letters. There are 8 vovels come in two subsets:

AIOU and EİÖÜ.

When you pair them with zip(), pairs are phonetically related sounds but totally different letters at the same time. Turkish also uses suffixes for everything, and vowels in these suffixes sometimes change between these two subgroups.

This design lets me write any word uniquely and almost correctly using the Turkish alphabet.

Dis dizayn lets mi rayt ani vörd yüniğkli end olmost koreğtkli yuzing dı törkiş alfabet.

Ö is the dotted version of O. İ is the dotted version of I. Related but different. Their lower case versions are logically (not by historical convention): öoiı. So we didn’t just wanted to change I, and only I. We just added dots. Since there are no Oö pair in any language our OoÖö vovels didn’t get the same attention. Same for our Ğğ and Şş.

I hope this answers the question.