Pro Sites upload quota when downgrading

What happens when a site is over quota but they want to downgrade their plan?

Example: A site has uploaded 2 gigs worth of files in their Pro site. They don't want to pay for additional features but still want to keep their site.

Can they downgrade to a plan with only 500MB upload space, even though they are at 2 gigs?

Do they have to delete files?

Will it not even allow them since they are over the space limit?