Moving Current Subscribers to Free Level 1 for a year

I am getting ready to relaunch a site and would like the current members (8800) to get a level 1 membership free for a year.

I’m a bit confused on how to do this. I don’t want new subscribers to have access to the free level 1 –

What’s the best way to give this to our current list only? After a year, they will be paying the membership fee.
Thank you! Theresa