well, if it is just editing borders for cosmetic effect, then you can to a certain extent change the lukmap and *hey presto* - but if you are after doing a real job, apart from the (4 or 5?) lbm maps which need editing, you will need to edit the following things in the early.txt (or late or high) text files to do a proper job...

Province Origins list
Castle Positions list
Province border list
Province neighbour attributes list
Province port positions list
Province attributes list
Province Ownership list
Province resource list

and thats assuming you are not changing the factions...

as Krae said, it is a big job not to be undertaken lightly