Correction: RM-1073 is the hardware model number. The determination of what ROM it loads with is the product code which looks like: 059xxxx
AT&T has a different secure boot certificate which prevents loading other ROMs for the same hardware. Other variants use the generic certificate which allows flashing to a different ROM for the same model. There is a trick wherein you edit the registry to make it think it is a different product code, then let it download a newer firmware that overwrites the AT&T firmware. Unfortunately, for newer phones there might not yet be a newer firmware available. There are only a very few models that this trick has been successful, including the 635 gophone.