From what I've understood they will give you up to the full value of your ticket towards the AP within 14 days of the first use of your ticket (unless you have the no-expiration option). Basically, you can upgrade on your last day of your vacation without losing the value of your ticket...just don't wait past 14 days!
For example, an regular AP will run you $531.44 and your 7-day MYW+PH will run you $320.57. The difference being $210.87. Guest relations at any park or DTD can take care of this for you and get you upgraded.
We're planning on doing this w/ DW's 10 day MYW+PH pass on our upcoming trip.