date Posted: Jan 22, 2008 2:57 PM
There isn't an IN MOVIE explanation. They didn't address it in the movies.
As for the extended universe, I don't recall one, but that doesn't mean there isn't one.
I wouldn't be surprised if either Anakin made the change himself, or after a battle, new equipment was used. Based on some of the EU material, it appears that Vader occassionally got banged up pretty good.