That's what I planned to do and wanted to make sure I could do that. I don't care about getting pro rated for the 3 weeks I don't use it when I downgrade. I just don't want to pay $70/month just to have a backup on my own desktop since I won't be using any of the other benefits of the unlimited plan other than I need a backup.
I think a good idea for both you guys and all of your customers would be to offer for a fee a backup. I think any level customer should have the option to back up their work especially if it's saved only to their own desktop. I have about 230 categories in my store. If it blows up there is no way I'd start over again. I'd lose all my work, you'd lose a customer...everyone loses. For 10 bucks or whatever you could offer a one time backup for non unlimited plan customers. Just a thought that I think would be fair and putting the seed in peoples' heads that they should have a backup would probably make you more money in backups then you'd lose in people who don't need the unlimited paying for it just for the backup.