PayPal Express vs. Register Form

I need help on the following please:

I have set up the PayPal gateway so when the user click on subscribe they go to PayPal website and pay but I would like to know how to redirect those users after the payment back to my register form or even better have the register form page and in the end (after they fill up the info) they would be send to paypal to do the payment.

I also can not see how members can change levels of membership for example I would like that a free registered member has a chance to upgrade its current level of membership, I built a page inside of the register area but the paypal is not there, how can I enable ?

to have a feeling of the website flow:

Landpage (with a sign up button) then when they click this button, they go to membership type page (where they can select 4 choices) one free and 3 other paid , if it is free , they by pass paypal and got to the register fill up form if it is paid they go to PayPal , but I want make sure they come back to this register form (after payment)