Compatibility between legacy (R500) and next generation (R550) APs in unleashed

[beni_mahler]

AFAIK the first Generation of Unleashed APs (R500, R310) support up to firmware 200.7
The new ax APs R550 will not be supported below 200.9
Regarding the fact that all APs in a deployment must be on the same Firmware: Is iit true that very old and very new APs will not be supported in the same deployment?
Could or should we just create a new unleashed network running parallel to the existing one when we set up a new ap in an old deployment?

Any other ideas?