I generally won't do this, as this is something you should still be able to buy directly from Tesla. I don't do any unlocks or upgrades if they still officially offer it. Like, I won't unlock a software locked battery pack or activate the FSD package for people.
On a salvage vehicle, though, all bets are off, since Tesla refuses to touch them anyway.
Now if they decide they're no longer going to offer P90D to P90D upgrades themselves, that's another story. For most 90's it's mostly software and a physical verification that the proper parts are installed. If not, install them (proper pyro fuse, mainly).
Indeed, they no longer sell an upgrade path from P90D to P90DL. Confirmed via the attached email.