That's a tough call.

I know the mature answer is to be the better person and do it anyway, but let's be honest - do you think the other person would write a fic for you?

If you like the challenge and feel it would be something you would be interested in writing, then do it. If you are really that unsure about it, ask to change with someone else. It's early enough that you should be able to.

Good luck either way.