It is poorly documented by Garmin but you can have multiple maps in the "Garmin" folder on your microSD card as long as they end in .img. You can have up to 4GB of maps. They will all show up and you can enable/disable them as required via the Setup/Maps function.

This is certainly far easier than removing the microSD card every time you need to change your map region. I personally found it highly confusing with all my maps named gmapsupp.img. At least now you can give them a sensible name such as USA.img etc.

Took me a while to find that out but it's great.