VM can be vMotioned to new generation hardware, and vMotion back to its original host, as long as the VM has not been powered off on the destination host with new hardware.
When VM is powered off and powered back on new host, the new host EVC mode setting will taken into effect. If VM hardware version is too old , there could be kernel panic, at least for Linux OS. Guest OS might prompt user interaction in VM console to confirm regular boot process, depending on vendor, then guest OS will start successfully. However after this power-back process, VM could not vMotion back to its original host anymore, since EVC setting is working for the VM. EVC compatibility prevents reverse process.
