Sorry to revive the old thread. But I thought that I should give an update on this.
I got my new memory card and it now works flawlessly with all the old files. This is what I did:
1. Connected the phone to PC via USB.
2. Make sure the Hidden Files are visible on your PC.
3. Copy all the contents of your old SD Card to a temporary folder.
4. Use a memory card reader (or Insert it on some other device, but not your phone) and then copy all the files from that folder to your new memory card.
5.Once its done, switch off the phone, replace the SD Cards, and it works as if nothing was changed. My maps are intact.
Good luck.