CF has said before that it can't be done. In general, you can't upgrade a ticket that has one kind of entitlement (i.e. a major park day) that has been used to another ticket that doesn't include that entitlement.
For example, you can't upgrade a MYW w/WPFM where you've used a WPFM option to a standard AP, you have to go to the Premium AP which also includes the water parks/DQ, etc.
And you can't apply the GAD voucher to a water park ticket either...it is ONLY good for a 1-day admission to one of the 4 major parks (or one of the alternate options if you have a multi-day ticket already).