Board Thread:Game Discussion/@comment-9227192-20130507121959/@comment-9099835-20130507124318

On #2 above, certain cards add more experience when upgrading, but then it is usually tied to how much it costs you to upgrade the cards. Using 8 r2 cards will upgrade more than using r1 cards, but it will also cost more credits.

As others have said, usually most people will sell old r1 cards for credits, then use r2 cards to upgrade.