Excel formula problem, Auto increment number with extra character

I'm trying to fix a spreadsheet that I have, it used to work like this:
In one cell there is e.g. I001 this cell repeats every page on one sheet and copies from the previous one I001+1

But for a while now it doesn't do it anymore, I can’t find the formula or macro that did it for me, when I enter a plain and simple =A1+1 or similar I don't get what is supposed to be I002 if it was I001, I know the problem lies in the ''I'' as the calculation only works for number, is there a way I can include the ''I'' in the calculation without getting an error or having to use a second cell just for the ''I''

Basically if I put I001 in cell A1 cell B1 must automatically Be I002

There are roughly 50 that must be done like this in +1 increments from the previous one, and due to the amount of time I have available to me just manually entering the new number every time for 50 cell’s is not an option