I've been considering changing up the rules a little bit on just how crystal modifications apply. In the current rules, once you attempt an upgrade, it's permanent, whether you succeed or fail, and it applies to anyone who picks it up. Now, that's fine for a blaster or something, but lightsabers are personal weapons, and if you botch a couple important upgrade rolls, you might want to go find a new crystal and start all over.
With my tweak, each crystal is attuned to a specific person. Only that person benefits from the crystal upgrades (my group just finished Chronicles of the Gatekeeper, and so they have the fully upgraded lightsabers of both Suljo Warde and one of the Jedi from Cato Neimoidia, both quite a step up from their own). To take true possession of a lightsaber, the new user will have to reattune the crystal to herself. Doing so clears all of its upgrades (even failed ones!) and gives her the ability to assign her own upgrades at the reduced difficulty.
Note that the original owner is also allowed to reattune his lightsaber, and will generally do so to clear failed upgrades, after increasing his skill and Force Rating, so that he can upgrade it completely.
The Difficulty would be based on how attuned the crystal is, how powerful the original owner was, if his morality opposes the new user's and whether he's still alive. Trying to reattune the Emperor's lightsaber is going to be very difficult, and may well act like a beacon to your location, if you roll a despair.
I'm on the fence about whether the lightsaber's upgrades should only work for the one to whom it's attuned. After all, Palpy's saber should be terrifying in anyone's hands. The other option would be to waive that rule (but you still need to attune it to yourself to upgrade it), but in that case, I'll just never hand out fully-upgraded crystals.
Thoughts?