Of course he's allowed to change it. Anyone can perform it anyway they want. I'm just saying it's not necessary IMO as it is a piece of art. I can also choose to dislike the change. If they go through plays that have offensive material and take it out for the sake of the current P.C. climate, it can completely change the point of a work.
Admittedly, the song is no where near as deep things found in other art forms. But for example, if the word "fuck" wasn't in the end of Catcher in the Rye on the wall, the end wouldn't have the same impact. I think the word "retarded" in that line completely captured where he was at that point, and most importantly the immaturity of the speaker (who is presumably Mark himself). This was a huge theme of the song. It's fine that older Mark wants to change it, but I can still not like it.