Add Volcano Bay to UT park-to-park tickets after arrival?

[CyndiLooWho]

I am buying 4 day park-to-park tickets for an August girls trip for 3 50+ something gals. I don't think we are going to want to do the water park.

But, if we get down there and change our minds, can we upgrade our UT tickets to add Volcano Bay? And how will the purchase price be credited?