we have also changed an mmsi with the aid of instructions over the phone on a boat we delivered back here from Ireland. The previous owner had indeed programmed a number in but I doubt the mmsi 123456789 was actually legitimate
There was a thread on here many moons ago in which a guy did describe how to change the mmsi on various navman models but it seems to have disappeared into a black hole somewhere.
If you wouldn't mind colcreate
could you pm me the basic instructions, or where I can find them, for Navman radios as the one I have put in the rib is out of another previous one and still with an old mmsi in it.