Not sure if a new map automatically becomes the 'current' map, but you can turn maps on / off in the Nuvi menus, This the menu for the Nuvi56 so it may be slightly different for you:-
Main Screen >> Settings >> Map & Vehicle >> myMaps >> [select / de-select from on screen menu]. Note when different maps are of the same area, I would only select one at a time, but when the maps are of different areas, it is probably safe to have them both enabled at the same time e.g. aust/nz and USA. You can test the map is selected by zooming down to street level in the area you want to go with 'View Map', if the new map is not working, you will only get the world base map which will only have main roads
