That's exactly what happened with 6750 and 6770 cards :|

They are literally 5750 and 5770 Juniper chips rebranded as a 6xxx series card, only the BIOS is different.
http://www.anandtech.com/show/4296/amds-radeon-hd-6770-radeon-hd-6750-the-retail-radeon-5700-rebadge