Replace substrings but keep constant overall string length

I need a formula to replace a character that may or may not repeat itself in a string. I've read through enough threads about FIND/SUBSTITUTE to think there's a way to do it, but none of them help me keep the overall string length the same.

Example - let's say I want to replace the * in the string with # instead. ABCD*FGH becomes ABCD#FGH. I can handle that. But my source data has varying numbers of *, and string length must remain at 8 characters, and FGH will always be the last three characters - that's where I get stuck. ABCDFGH are variable, not always those specific characters.

ABCD*FGH becomes ABCD#FGH
ABC**FGH becomes ABC##FGH
AB***FGH becomes AB###FGH
A****FGH becomes A####FGH

I know how to handle each separate scenario, but don't know how to nest the pieces in a way so one formula will address all those possible scenarios.

Thank you for your help!

If I am understanding you correctly, this should do what you're looking for:

=SUBSTITUTE(A1,"*","#")

Oh, my goodness, I feel silly. That worked.

I had been under the belief that that would only fix the first instance of * and wouldn't help when multiple * appeared in the same string, so searched and tried multiple nested SUBSTITUTE and IF variations.

Thank you.

You're welcome. That would have been the case only if you specified an instance number.

=SUBSTITUTE(text, old_text, new_text, [instance_number])