I think there might be problems with that due to many of the coordinates changing. In France we probably didn't change any, and thus overlaps between old and new worked.
That coordinates didn't change caused a problem for HDX. The MEX change is less critical here.
Changes like we are facing with MEX are always a hassle to users. The split of France was simpler than the previous split of Spain and Germany since we had yakra's tool. ESP+DEU list file entries had to be "converted" manually. Line by line... We had set up the new systems on tmtest server and I could compare manually to production server and figure out the new user list lines. Mexico will be more tricky since you cannot compare the total region mileages because the whole routes will change - if I got you right.