Disney places no value on plus options for the purpose of upgrading/getting a credit towards another ticket. If all you have left is the plus options, just keep the tickets and use the plus options on future trips.
The following is from WDWIG's Ticket FAQ:
How does WDW credit my old passes towards new ones?
Walt Disney World credits are based on what you *paid* for the ticket that you are exchanging. Example: If you have an unused one-day ticket that you paid $20 for then your credit will be $20 towards your ticket purchase.
For multiple days, the credit is based on the number of days of the ticket, the number of unused days and the total original cost of the ticket (though it has never been completely verified it is close to the actual formula of unused days divided by number of days times original cost.) Example: You have a 5 day Park Hopper pass you paid $200 for when you bought it. You have two unused days left on it. You would get a credit of 2/5 of the original purchase price. In this case, it would be $80. Note that Disney does not apply any dollar value to unused Plus options on Park Hopper Plus passes. Only unused major park admissions carry a cash value when upgrading.
Applying the credit from old passes can only be done at a Guest Relations location at one of the major parks or at either of the two Guest Relations offices at Downtown Disney or at Guest Services at your resort.
