Need help creating the following user experience

How can I achieve the following user experience using the following tools:

- WordPress Multisite
- Gravity Forms
- User Role Editor, Menu Editor, or Adminimize

At this point, I'm not really interested in ProSites (other than offering premium plugins / themes), as the default pages look really weird on my site.

Here we go:

1. The user selects one of three plans from the price sheet.

2. The price sheet sends the user to one of three forms, which are unique to one of the three plans available.

3. The user submits the form, inherits the default user role, and is automatically logged into their sub-site.

4. User wants to upgrade their plan.

I think I'm on the right track with all of this, but can't seem to connect the dots and make it all work -- especially with step 4! I don't understand how to give users the option of upgrading without prosites.

All ideas welcome!!!

Thanks : )