If you goto the sticky on the theme parks board "everything about WDW tickets" I think on page 2 one of the first couple of posts is about the GAD promo. You do have to use the ticket before it can be upgraded, and the ticket is only valid on the day you exchange. After that it can be upgraded like any other 1 day ticket. You will have 14 days from first use to upgrade it as you would like.
So you would
a) exchange voucher for ticket.
b) use ticket at a park on that same day
c) upgrade as you would like within 14 days.
d) enjoy upgraded ticket
The direct link to the ticket area about the GAD is here
Are you looking for 2 uses? To add it to the GAD voucher it will be $55.38 but you will get 2 uses. If you are only looking for 1 use, I would buy the waterpark ticket outright. That would cost $47.93. Makes me wish they still had Pleasure Island.