how to give multiple options to user after free trial

We are trying to setup a process where a user has a 7 day trial (finite) followed by the option to sign up for 1 of 3 paid levels.

How do we set this up? Do we put all three levels in the subscription or leave it just as 7 day finite? And if so, how are they offered the option between our 3 paid accounts?

And if they are already a user how to they sign up for a new subscription without having to create a new user?

Essentially we do not understand (and have not been able to get a clear answer) how the user moves from one plan to another (ie., what physically happens, where are they directed, etc).

Thank you!