Just wanted to agree with Dean on how the passes are valued.
I had a slew of old passes and a couple that had not been used that I purchased thru TicketMania and 2 AP Vouchers I received for our SSR add-on.
The only way to check how many days are left on a pass and the residual value is to take your passes to WDW Guest Relations in person. They cannot tell you anything over the phone. (and I mean this in a nice informative way

)
The AP Vouchers I received from my Addon @ SSR were worth face value $379+Tax because they were purchased directly from Disney and given to me as an incentive.
I purchased the special 5+4 hopper passes from TicketMania last year. I paid $270.00. When I checked the value of these unused passes at GR in DTD, they were only worth what Ticketmania paid Disney for them (not what I paid) and that was $255.30 a loss of $14.70.
These passes had 4 plus days on it and as stated, they are not worth anything when upgraded, so I decided to keep them initially.
We were at WDW for a month and after my PAP expired mid trip, I later used 1 plus on my 5+4 at Blizzard Beach.
I returned to GR@DTD as I was contemplating renewing my PAP with the
DVC Discount and since we had used days etc. checked my leftover passes again.
I had used 1 plus from the 5 day pass, and that only decreased the value by 50 cent, so in theory, I could have used the plusses up and then traded in the ticket with less than $3 loss in trade in value so the plusses really are close to free.
I instead, traded it in for a 10 day+5 and didnt renew the PAP because my Dh and boys would much rather do DQ and WP then go to the parks.
We actually spent more upfront than we would have if we purchased the DVC PAP, but based on our touring style, we would have wasted the park days and mainly used the PAP to visit DQ and the WP's. We were there for a month and visited theme parks a total of 5 days (and 3 of those were US/IOA).
So we purchased 10 day hoppers no exp. for Dh and the boys for multiple trip use (should last 3 years) and then DQ/WP AP's (which we used daily) that we will renew annually and is an excellent value considering DQ closes at midnight or 1 am on a regular basis.
If you have lots of old passes, take a pad and pencil and a calculator with you and take your time to figure out the best scenario for your family. That's why I liked the DTD GR location, very low key and they didnt mind giving me the costs of the different passes then me going to sit down and figure out what would work best for us. I was up and down quite a bit with questions and they were always helpful and understood.
It's a pain that you can only check it there and can't do it from home but it is best to take your time because passes are a major purchase and with DVC, you have to consider future trips and timing.
HTH!