In this case search would become somewhat clumsy. In TeXmacs one can search separately math and text (I am testing with 2.1.4 on Ubuntu but this feature is around since a while). If you want to change your i indexes, searching in math will find them if they are written in math mode; there is structured search as well, which I explored superficially, and might help you to look for i in subscripts only.
If you use text mode to write indexes, a search for i in math won’t find them, and a search in text will find all of the is in the whole text; again structured search might help by getting to search only text in math subscripts (it is a guess, I did not try), but standard syntax will give you simpler search.