What happens to upload space when user unsubscribes?

Hi,

I'm using pro-site plugin. What happens if the free upload space is 20MB and the user upgrades to a Pro account with 10GB of upload space, and uses say 3GB of that space before deciding to unsubscribe?

His account goes back to being a free account I understand, but what happens now that the user is using more than the allotted space for free accounts?