Migrating existing sites into Pro Sites (billing)

Pretty sure I have Pro Sites geared up and ready to start using. New subscribers will sign up on the site and get added through the Pro Sites system. Correspondence will be automated in the event of non-payment, expiration, etc.

I have a bunch of existing client sites I'd like to migrate in as well. But I'm not sure where to put em. I want the same automated correspondence as everyone else, but these clients are already loaded up with a different payment gateway (authorize.net...evidently not compatible with Pro Sites).

Here's my question: Should I load these clients up to a new level just for existing clients (Legacy Level, for example)? Activate manual gateway and when I load them up, designate their specific timeframe before they'll have to convert over to the Pro Sites payment gateway?

In other words, I'm thinking of doing this:
1. I create a level named Legacy.
2. Load up existing clients and give them access to this level.
3. If an existing client has 3 months left on their contract, I would give them 3 months of access before they have to renew. When their renewal is up, they'll simply process payment via Pro Sites interface, and they'll be good to go from there.

Does this approach make sense? Am I missing anything?