Also, if you call the toll free number on the back, they can combine them. You may need your original receipts and activations reciepts.
If the dollar amount is more than what's left On the card, you will just have to pay the difference.
Also, California law states that if you have $10 (I think it is $10, but it might be $5.) or less left on the card, they have to give you the cash for the balance if you ask.