No, that’s not retroactively changing the terms. If you don’t want to send players under their new terms, then don’t. Noone ever agreed to always offer you the same program forever.
The fact that they have readily agreed to honor the old terms for the old players is to be applauded. Too many times now we have seen programs have to be dragged kicking and screaming into compliance with their original terms. So far, RewardsAffiliates appears to be doing the right thing.