Garmin 800 SD card maps onto 820?

Hi everyone,

Just upgraded from my older Garmin 800 to the 820. On my 800 I have the Garmin OS maps on SD card. Just wondered if it is possible to copy these onto the 820? I've tried copying what I think are the correct files but I get a message about can't unlock maps when I restart my 820. I'm guessing they are locked to my 800 or aren't compatible.

Checked on the Garmin website and it seems I can download OS maps for the UK and it looks like there is an 820 option (although they offer download or SD card????) but they want £199 for the privilege which seems crazy when I already have them on my 800. Use my Garmin for both on and off road navigation so really like to use the OS maps.

Is there anyway to do this?