If a production alters an prop then it's still part of it's original intended purpose, even if it's re-use for a sequel or whatever since they still own it.
Once an original prop gets into the collector's market it's a different story because the item has the distinction of having been in some film or show and people want it for that reason. The new owner then is more of a caretaker of the piece because of what it is. He owns it, but it's not really his to do whatever he pleases with it. It's his responsibility to look after it and preserve it, not to alter it or trash it.
The ANH Stormtrooper blasters were refitted with replacement greeblies and parts in order to bring them back to their original look, but actions like that are reversible. Painting authentic Stormtrooper armor is not reversible which is why it shouldn't be done.
It all depends on the piece because correctly restoring and repainting things like foam latex masks (a la Tom Spina Designs) is done for preservation purposes. Items need to be judged on their own to determine what the correct action should be.
I realize many folks on the RPF are very much fans of wearing and repainting, but items like this aren't R- they are O and that is a world of difference.
Repairing cracked armor or adding in a replacement for a missing component is one thing, but repainting is severely detrimental. This isn't a homemade Halloween costume, it's an original costume from one of the most famous and cherished movies ever.
Just enjoy it as-is then when the next caretaker has possession he can continue to maintain it's integrity.
-chris