With a brand like Kohler, which changes parts more often than I change my underwear(!) it is likely you would have few if ANY choices regarding upgrade to an old rough=in valve. You are much better off going all new.
WHich brand? Obviously, your Kohler has given you way beyond an expected life of about 20 years for lavs and showers.
I have liked American Standard and Delta.
Personally, I stay away from Grohe, KWC, etc. or other "foo-foo" brands which are gorgeous, but where will you ever find parts?