It was finally easier than I expected.

There was already a gmapsupp.img file in the internal memory of my gps. From the size of it, I guess that it was the map of Romania I had recently added.

So, what I did was to copy gmapsupp.img file of Bulgaria to the extra card that I have. That was it. Now I have all the maps that I need for this trip. No renaming was done as one file is placed in the internal memory of the device and the other is placed in the additional sd card .