WDW ticket price increases generally bring about questions as to how much a ticket is worth when you upgrade it at Guest Services. Here is my understanding of how things are currently working:
If you have purchased your ticket from WDW, you will get a credit for the Gate Price of the ticket at the time you purchased it.
If you have purchased your ticket from a third party reseller or your ticket was part of a vacation package purchased from WDTC, you will get a credit for the current Gate Price of the ticket.
Note that in order to upgrade your ticket you must have used at least one park admission and you must have park admissions left. You can also only upgrade to a ticket that hass more features (i.e. park days, PH, WPF&M) than your current ticket - you cannot drop any features even though the ticket you are upgrading to costs more than you current ticket (e.g. upgrading from a 4-day WPF&M to a 10 day base ticket). The one exception to this is upgrading to an AP - if you have a WPF&M ticket and you have not used a water park admission then you can upgrade to an AP.
With all this in mind, because the resellers like UT and OTC have not increased their ticket prices yet, you can score some sweet deals on tickets that you can then upgrade to APs. The best deal I have seen is getting a 4-day PH+WPF&M (+ 1 day free) from OTC for $360,06; if you upgrade it to an AP you will get credit for a $431.33 5-day PH+WPF&M ticket, meaning that you will save $71.27 on your AP.
ETA: Whenever you are going to do a ticket upgrade make sure that you know exactly how much you should pay. Some CMs are not as knowledgeable about price bridging as others, so be ready to ask for a Supervisor if the numbers are not coming out right.