I checked this in the manual and it looks like the right approach. I'll try it when I get back to my boat this weekend. Thanks all for your helpBy default, Multigraphics are configured to be the master, meaning you are probably introducing a second master display into your network.
Only connect the power but not the bus pin (white and silver if I remember correctly) so your new display starts up but is not connected to the bus. Then go into the settings and change its address. Once that is done you can shut down, connect all wires and start everything back up.