Migrating from EAP 5 to EAP 6

Hello,
We are migrating from AS 5 (EAP5) to AS 7 (EAP6). While doing so we are supporting defects and emergency releases. Decision is to have both instances on the server box until process is complete. So, any possible defects will be with the old, AS 5 while AS 7 at that time will not be running and vise verse otherwise. Possible issue is that assumption is AS 7 will require new Secure Certificate installed. My question is will that be possible to use new Secure Certificate for the old instance as well?
thank you for the help.
Al