In the U.S., under FCC regulations
,. only a licensed tech can change the MMSI. I had a Garmin 600 which I sold to a friend. The local dealer charged $60 to change it. I bought a used Vesper. Vesper customer support told me they could help a non U.S. customer change the MMSI, but couldn't help a U.S. customer. Another $60. Not an outrageous amount, but annoying.