How to do this with an iPhone?

Okay, so here's what I want to do. I've been sharing an iTunes account, library, and computer with other people. I want to get a separate iTunes account and be able to use it on my iPhone and a new computer. Here's the catch, I want to keep some of the old music and still be able to sync the iPhone with the new iTunes account. Is there a way to do this?

I don't think so, I think you're stuck with the joys of Apple's DRM music protections.