How to keep free users accounts as free after activating pro sites

Hello everyone

I want to start my network as free for everyone, later on I would like to start a paid options "Pro" for the new signups, my question is:

How do I keep all early users as free with all the features of Pro, and new users Pro have to pay for the same features.

Meaning Early users would get a free lifetime account and New Users have to pay.

Those that make sense? Should I do it this way, any recommendation would greatly appreciated.

Thanks
W.