Paypal payment through IPN

Paypal and automated payments? I want my visitors to be able to type in their members id in a field and if payment goes through it updates their account to a Premium member automatic. Just like this url one [login to view URL] just like that except the category list which is not necessary on my website. So if you have been working with paypal and ipn a lot this might be the task for you. Its nothing fancy just very simple. Best Regards Fallzone