I used a III+ for years. You can update the maps. In fact the map memory was so limited that I had to create a new mapset for almost every ride.
I was able to upload routes to mine. I was only using Topo 100k at the time, so I don't know if the unit supports routeable maps.
You have to be really careful with your via and waypoint names since the unit truncates them to 8 characters and only saves the last one loaded.
And I don't know for sure if you can upload tracks.