This procedure works for me, with a Go910, SE Navcore 9.051c, and 895 Maps.
The trick is to have in the tomtom two or more maps.
Only one map is the active map (the last used map) the other maps are sleep maps.
If you copy your old mapsettings.cfg (Favorites and more) to the active map then the unit don't start, BUT
if you copy your old mapsettings.cfg (Favorites and more) to one sleep map, the unit can start with the active map,
then you can switch to the sleep map, this convert this map in the active map, and is VERY important to add a new favorite (the mapsettings.cfg is rewriten). This is all, then you can start the unit with the new map and old favorites.
Hope this can help someone.
P.D. In a Go910 the is no problem to have as many maps as you want, but if you have space problem, you can use as a second map one like morocco_890.4224 <8MB making it as your active map, and your map as one sleep map.