I'm pretty sure it's the same as Xbox Live, wherein if you don't renew it, it reverts to a "silver" account that can be re-upgraded at any time.
If you have a premium account and you submit a song and it gets published, you should be fine if you choose not to renew your account. As a non-premium member you would still be able to post in the Creators forums but would no longer be able to access any songs that are in Playtest/Review - although someone who buys a short-term XNA account solely for the purpose of getting a song through the pipeline probably wouldn't care about that, anyway.