Went through this with Dropbox when I lost space provided under a time-boxed promotion. I could keep data that was already in my Dropbox, but I couldn't add anything more until I dipped below the new limit.
I expect what Amazon does: Amazon just let me keep the files, but won't let me change aanything when they removed their free ofering.
Dreamhost had 50GB of backup which they no longer offer, their file permissions were so mangled you could traverse and download stuff from a good portion of their clients.