Is there a membership plugin that doesn't require a new signup process?

Like many others here, I'm trying to create a new site-builder, but I'm having trouble with one particular aspect.

THE SETUP

I'm using Gravity Forms (with the user registration add-on) alongside an install of WPMU -- everything seems to be working well, as people can select a plan, register and create a site using the form I've created.

I would like to offer at least two plans -- one free, one premium. I plan on using User Role Editor and Menu Editor to customize the access and presentation of the backend of WordPress to accommodate each respective plan.

THE PROBLEM

Is there a membership / site management plugin out there that enables easy control over access based on payment ... one that I can install without having to change the sign up process?

I have access to Membership, ProSites, WishList Member, and I'm willing to purchase anything that might help me achieve this without changing the signup process.

Ideas? Questions?

Thanks!