If you don't have (or don't use) iFly Streets, yes - it's pretty easy. You just need to copy all the contents of your 8GB card to 16GB.
The Need more Space alert is to address a bug in the old update app. We require a minimum amount of free space to perform the update - packages of files are copied to a temporary location, validated, then deployed. We try to keep the max size of a package relatively small (couple hundred megs), but sometimes they are bigger so we needed to increase the min-free-space threshold on the card. So if you are right on the edge of a full card, you will get this error after updating to this version.
On another note, when version 9 is released there will be a slight increase in file size to support seamless charts. Again, not dramatic - but something you may want to be aware of if you need the entire country loaded. This will be a bigger issue for users with a 4GB card, or 8GB with Streets. We are also adding an option for our IFR customers to download High-Enroute charts, which will also require more space.
So, if you don't need the IFR, or if you are comfortable dropping parts of the country from your database, upgrading your card is not needed.
But if you want a turn-key solution for upgrading your card, we have 16GB cards in our store for $35 - and these come pre-loaded with all the latest software and charts.
Thanks,
Walter