Hi Con,
from a driver and software perspective there is absolutely no difference between an ESXi install done on a HP Gen6 server and another one done on a Gen8 server - if you use the same installation ISO in both cases.
However, the configuration data stored on the system will be unique to the machine that you used for the install (e.g. there are mappings done from the MAC addresses of the physical NICs to device names).
The media will still boot on another HP server, but at least vmknic and IP address assignments will be changed, and the configuration will have orphaned entries for devices that were present on the install system, but not on the target system.
Therefore I recommend that you use the "Reset System Configuration" function from the DCUI main menu to reset the system configuration data (sic!) after you boot an ESXi system on a machine that was not used to install that system.
Of course you need to reconfigure all settings after that, but you will then have a "clean" system again.
- Andreas