Depending on your party's makeup, FD may be a better deal than the 5/2 currently offered for the value resorts. The free night deals have more "value" at the higher priced resorts.
You can't combine deals, but you can change the promo if the pricing works better for you AND if the rooms are avaialble for the promo. They will issue a refund to your credit card. However, since there is a reduction in price after the 45 day final payment date you will be charged a $50 change fee, and if you are changing within 14 days of arrival there is an additional $10 fee. They've told me it's for docs re-issue/last minute booking. You really don't need new documents, but they will automatically re-issue them anyway.