From Intrepid,I have the same problem but with 1:25K mapping.
I've owned a 3500 for just over 12 mths and it came with pre-loaded 1:50k Full GB mapping and the relevant disc
In order to fit more maps onto it, I've tried to replace my micro SD card from 4Gb (as supplied) to 16Gb and as expected it was a disaster
(I know there is reams of forum stuff about the perils of doing this but it is all a bit too "geekie" for an oldie like me).
Now when I put my old SD card back in to my 3500 it prompts me to re-activate the licence key by re-entering my Serial No which it then tells me I have previously entered which seems to be "stale mate"
Can anyone please explain what I have done wrong and how I can get things back to as they were?
Help would be appreciated