I did experiment with moving characters between maps.
If you copy the .../Saves/<Map>/<Game>/Player/999999999999.ttp files from the old map to the new map (similar path), your character arrives complete with their inventory. Their location is where they were on the previous map (N-S, E-W) and you seem to start on the ground (elevation) even if it was different from before.
I've no idea what would happen if your N-S, E-W position happened to be in a POI. That didn't happen in any of my tests.
One side effect was that you had all of your map waypoints from before, but each player could just delete them.
This kind of makes it possible to say "if you want to move to the next map at the end of the month, you need to get to this location" which would work with my small group, but probably not on a server with a larger player base.
I find myself wishing that there was some convention like highways always entered the map in the N-S center and E-W centers, which would create some map-to-map continuity.
I would certainly appreciate any features TFP might consider making map-to-map travel possible so a server could switch to a new map and provide some continuity to the characters/players. Also, could you image, perhaps, a couple of servers going together and there being some kind of hand-off between them?