Gentlemen, a successful update report here.
My PartNo on the original SD card is: 6P0919866H
I managed to update from 0635 to 0820 with exciter's approach, i.e.:
1. Reformatted the SD card that had 0635 on it
2. Copied the freshly dowloaded 0820 version to the SD card
3. Replaced /maps/EEC/EEC_WLD/OVERALL.NDS on the SD CARD with that from 0635
4. edited the /maps/00/nds/dbinfo.txt so that one of the PartNumberX entries correctly has my own part number in it, that is I replaced PartNumber4 from PartNumber4="6P0919866R" to PartNumber4="6P0919866H"
5. Inserted the card into the card and voila: 0820 working.
Step #4 may be completely useless, I did it nevertheless, and did not test without it. YMMV.
The fact that exciter made it work without #4, is a strong argument to try without it.
This is a fresh: tested a couple of route calculations, searches, all was OK so far, no reboots. Will report back after a couple of days.