*PLEASE READ* Website subscription payment levels via PayPal

I have a website, for which users can sign up and subscribe. At the moment, I have only programmed one subscription level. You will help me to create several subscription levels. Assume a user can pay £0, £1, £2, £5, £10, £25 (not correct figures) per month. I need to create the buttons and payment scenario that allows the user to sign up to any of the packages. (Lets call them, A, B, C, D, E and F (A being £0, F being £25 per month). The problem I have is how PayPal reacts to changing subscription. Suppose mid way through the month, a user requests to increase from package C to package E. I need PayPal to increase their subscription for future months, but also charge the difference between C and E packages pro-rata for the remainder of the month. If a user cancels from package F down to B, they will simply be billed less from the following month. I also need a protocol for failed payments and subscriptions cancelled within paypal.com. And an option to cancel the subscription via the site. The rest we need to discuss how this can be achieved. I need to know the feedback to the website for each scenario, change of subscription, cancellation, failed payment, etc, so I can alter what their account does with this information. Each user has a unique ID, and a unique token for security which need to be passed via paypal. As well as any transaction info. If you truly are familiar with PayPal, this should be quite simple. I have looked at the developer side to PayPal, and would rather someone competent with experience assist with this. For filtering preferences, please start you proposal with [login to view URL] in capitals.