There is no catch. Smaller websites using fewer fewer resources can use the SiteApps platform for free.
Yes, you can upgrade or downgrade your plan whenever you want. The change will take affect immediately, and you will be credited an amount equal to the current month's remaining charge.
SiteApps charges per website - thus, your account may have different websites with different plans.
We'd love to help out, simply send us your question here, and we'll reply as soon as possible.