Do you have a map in the card that you want to use in the "new" gps or do you just want to transfer a new map to the card?
If the second case you just have to unlock the map in Mapsource and transfer it to the card. It will overwrite the old map in the card.
I do not know if it is possible to transfer the map from the card to Mapsource, have never tried that though I have the old type of memory card in my gpsmap 176 and 276C gps.
If you can transfer the map from the card to Mapsource, then it should be possible to unlock it and re-transfer it to the card.
Info on how to unlock can be found in the forum, see the Sticky posts!