Plugin that upgrades a users level free

edit: I wasn’t aware of the paypal pro/free registration form. I will use that and see how well it works out for me. This thread can be disregarded.

Hi guys, I’m considering writing a plugin that lets users become a certain level if they complete a survey.

Basically my idea is something like this:
make the users register as non-paying customers first. Then, when they visit a link a userID is sent with them from the WP site onto the survey, and upon completion the ID is sent with a confirmation code back to a php (hook? script?) that updates the users level.

What do you think about that? Is it even possible to develop a plugin/use a hook to freely upgrade a users level? (without going through s2members payment system) (theoretically speaking – security is not an issue so far, it is more a prototype)