Firstly, I know very little about iGo, but would like to get hold of v8 myself - it seems to have be delayed.
A memory card is a memory card isn't it? The only difference being the size.
Have you tried copying all the files over to the new card using exactly the same file structure? You need to include any hidden files in the copy.
If this still doesn't work check out that the name of the new card is the same as that for the old card.
I know that this method works fine with TomTom, so I would hope it works with iGo too.