Also, I would ask, what version are you running on your "current" gateways, and what version will you be running on the "new" hardware.
Example, currently running R75.47 going to R77.30, or running R77.20 going to R77.30, or R80.10. Or will they be the same version?
I've done this in the past going from Fujitsu;s to appliances as well as upgrading the version the gateways were on as the new hardware came into the cluster. Along with Vladimir's questions, I think some of these answers would determine a desired path.