Though quite commonly used indeed, there is no longer an officially supported way to convert an 48 bits MAC-address into an official EUI64. Also, the new MAC-address could already be in use, or belong to a future gateway, especially when based on the same hardware and when only making minor changes to the unique factory MAC-address.
While the chances for collisions within TTN might be very low, it might also be easier to use an officially supported random EUI64 instead, and change the gateway’s code (or configuration?) to use that value.