XF 1.3 User upgrade, how can I up the price for new members but keep same for older?

[surfsup]

I currently have 2 plans

Monthly
Annual

I would like to keep the prices the exact same for all past subscribers but have new prices for the new members to subscribe. I need to do this where no one else can purchase those previous plans but still keep the old subscribers.

If I 'uncheck' the Can be purchased box, doesnt this disable the subscription entirely for past members to or not?

I have a lot of subscribers and just want to double check that nothing will happen if I uncheck that box or let me know what would be the best way to do this?

Thanks

 

[Paul B]

Just increase the price.
Existing subscribers will pay the original price at renewal time.
New subscribers will pay the increased price.