I am working on a new project and was planning to use paypal's subscription option (at least to start off) for my users' monthly accounts. The thing is there are various plans at different costs. What if they get a plan that costs $10/month but then want to upgrade to the $15/month plan? How do you update the subscription amount, or cancel and apply a new one? As well, what if they upgrade in the middle of the month (billing on first of month). How could you charge the difference for the upgrade?