You can look at introducing a new characteristic CEKKO-BEDAT - purchase order date into your classification for the release class.This way you can segregate depending on go-live date if all you other parameters such as doc type, etc remain the same.
You should still have at least a one-step release for these legacy POs. It is likely that some of your legacy POs are going through date, text changes etc but it is also likely that someone could update prices, qty etc on these POs. Without a checkpoint you expose yourself to this risk.
Unless you have it as a process step to updates internal texts, no one should be updating legacy POs w/o a release strategy - these are valid legal documents that the vendor has agreed to and subject to the same protocols as a normal PO.