It's the unlearning that's the problem. You have to unlearn the original version before you can play it on the same instrument; playing a different version on a different instrument is just like learning a new tune, which is often easier.

It's the same if you try to learn a different version of the words of a song, perhaps so in order to sing it with a partner or an ensemble. The original version has to be deleted from your memory, otherwise it keeps interrupting and confusing you.

Marje