In Enhanced Edition games it can be done with the script action SetWorldmap("newmap"). There is no such script action in the original games though, so I don't know whether it's possible at all. TobEx doesn't appear to provide such a command either.
Thanks a LOT mate, it is a EE mod I am writing, ( suppose should have said that lol ).
As soon as I have uploaded it to github, I will have quite an announcement to make