Region 50 is for a small OEM map such as dealer POIs. It often shares space with region 49 anyway so if there's a map in there deleting it might give you a little more space in region 49. You can dump rgn50 with GPSdaemon to check its available space.

If you have a larger Garmin DataCard: The datacard is treated as a discrete region, that's region 10 and is for a supplementary map, gmapsupp.img. Again you can use WebUpdater in command line, substituting 49 for 10 and do it with the card inserted in the GPSMAP. Alternatively, if you have a Garmin datacard reader/writer you can simply load an img file from your PC to the card in a Garmin root folder named only gmapsupp.img (x:\Garmin\gmapsupp.img).

There's another method which is to load maps using Updater.exe to write an img converted to an RGN file to any of the three possible regions reserved for detail maps. [Only registered and activated users can see links. ]. That tutorial is for basemap replacement in region 3 but will also work for overwriting detail maps in regions 10, 49 and 50.

There's even another method and it's probably the easiest way for you if you have enough space on-board and that is to join the two detail maps with GMapTool and write that one single img file to rgn49 with WebUpdater in command line.