ls | while read filename; do mv "$filename" "$(echo "$filename" | sed "s/_/./g")" || break; done

I think there's a variable expansion syntax that does replacement, but I haven't learned it so I use sed.

I find that piping to a while loop is often simpler and more flexible than something like you've written, but maybe not in this specific case with the weird string replacement syntax. However, mine doesn't have a command injection vulnerability (that I know of).

A real scripting language can also do this easily:

for f in os.listdir("."): os.rename(f, f.replace("_","."))

Sometimes I'm tempted to make Python my shell. The fewer different syntaxes you need to learn for the same basic operations, the better!