as you know GPSMAP 276C may be transformed to 296 Avia
I tried to do same transformation of GPSMAP 478:
1. tried to rename 059501000450.rgn to 055801000450.rgn and upload to unit - error of updater.
2. tried to change firmware (using RGNtool for replace fw) - no success, unit power on with blank screen.
3. tried to copy FULLFLASH K9WAG08U1M from 496 to 478 - no success, unit power on with red screen "Software missing", when tried to update fw, unit return same ID and type GPSMAP 478!! (not of copied 496)
I think that firmware have protect of device type and check device HWID and Unit ID for run firmware.
My opinion HWID, type of device and unit ID store in Lattice PLD LC4032ZC.
What you think about this transformation - it available? What is the way to do it? (patch fw for replace PLD data to FLASH, patch fw for check HWID or hardware modifying)