Hi, Elisio! Your theme park tickets cannot be exchanged for a water park ticket, nor can you get credit for your 2 unused days to use toward a water park ticket.
You can add the No Expiration option to your tickets, but that would add about $110 plus tax to your ticket (prices subject to change.) With this option, you could use your 2 remaining days on a future visit. Otherwise, your tickets expire 14 days after their first use.
There is a bright side to all this! Going from a 5 day ticket to a 7 day ticket is only about a $6 price difference, so you really are not losing a lot of money on your tickets by only using 5 of your 7 days.
For more information and ticket pricing, click
HERE. Have a great WDW vacation!