They do it to books as well, but at least it's still the same book inside. Or is it? There is currently a lot of Web space devoted to arguing about the name of the wolf in the Narnia movie and, as usual, everyone is convinced that they are right. If you want to see the arguments, just search Google for both names together.
Every copy of the book I have ever seen has Maugrim, so that is the name as far as I am concerned, and I don't like Fenris Ulf, which doesn't sound right. (As far as I know, the one in Norse mythology was known as Fenris Wolf, but never mind, that's not the argument I want to have now.)
Fenris Ulf is apparently used only in US editions, and some people claim that Lewis made the change himself. I am not convinced that, even if the change came from Lewis, he was making a genuine revision, since the change never appeared in any British edition.
I suspect that a clue to the change lies in the words that appear on the back cover of my copy (Puffin Books, 1959 reprinted 1974) -
For copyright reasons this edition is not for sale in the U.S.A.
This makes me wonder if the change is just a marker for the US editions, in the same way that map publishers put in deliberate mistakes.