Song rights issues

Okay, so there's a couple songs I'd like to use in my next feature. One of them I've successfully acquired the rights to. The other one is going to be a little more difficult.

What if I just used it anyway? Not the original recording, but a new recording. What would happen? Is it just royalties that I'd have to pay? Or would I be setting myself up for a lawsuit?

Thanks in advance for any answers, and no need to add the "I'm not a lawyer" disclaimer. :)
If you record the song over, that makes it a cover, which can amount to legal repercussions.

There are several things at play when licensing music. There's a copyright on the recording. There's a separate copyright on the written music and lyrics*. If you make your own recording (cover) of the song without licensing/permissions, you're violating the copyright of the written composition and are setting yourself up for a lawsuit.

On top of all that - and this is where you need clarification from a copyright attorney - is a sync license, which allows you to use that song in a video or film.

*Unless the written music/lyrics predate 1923, in which case they're Public Domain. Recordings from that time may still be under copyright, though.