Using Tom tom maps on different tom toms

Hi

I bought some France Tom Tom maps a while back and work on my TT using version 5.45.

My sister wants to use them in her TT which is version 6 point something but all I get when I put the card in the reader is a little picture of a card with a line through it - suggesting that it won't work!

I asked TT they said visit the TTCode website and reregister it to my sister's TT which I did but I still had the same problem.

Any ideas how I can the maps to work on my sister's TT?

thanks.

Nicky