Restore vs. market value

[nydad]

Hi -first time poster:

My father has an old submariner, dating from c1970 or so. It's a bit banged up, he's basically worn it the whole time - scratched crystal, bracelet, etc.

I was thinking for his birthday to have it taken to a specialist, have it cleaned, serviced, install a new crystal, bracelet buffed, etc to bring it back to its former glory.

However, then I thought, i wonder if that will destroy some of the "vintage" patina and aspect that people cherish.

So, if we were to try to clean this watch up, but also retain its eventual collectors value, which things are ok, and which types of things should be avoided?