The license will remain same after upgrade to 7.x code. It is true that UR licensed PIX requires 128 MB of RAM for 7.x code. If you dont need UR features, you may contact licensing@cisco.com and request for a Restricted key, you'll need to send the "show version" of the firewall. As UR to R conversion is downgrade, you should get the activation-key for Restricted license from licensing team.
Alternatively, you may refer to following link for upgrading memory on your PIX-