It is not allowed to play musical masters among units once chart licences are cut against a particular unit. Due to contractual rules with the charting companies like C-map, there rules are in place to prevent any single licence from accidently running on several boats. Once a licence is cut, it can't be undone. The few exceptions are if the unit is fully condemned and being replaced with a unit from the same series, or if it is moved for functional mixed system like XL that would require the licence to be moved to the XL so that both TZ Maps can be purchased (for the XL) and MM3 charts for the entire mixed system. If no charts have been purchased, then easy you can change the master to any one you choose.
If the units are the same size you can always swap the unit location on the boat, if that helps.