The MYW no expiration PH can be upgraded to an AP at any time, not just within the 14 day window.
I keep stressing that Disney's policy on ticket upgrades has changed radically since the advent of MYW tickets and it continues to do so all the time. The current prevailing choice in the Ticketing Carousel of Policy encourages Guest Relations CMs to do as little dollar crediting as possible. To that extent, upgrades from used MYW tickets with no expiration first used more than 14 days ago to AP's are now being credited at full price and back dated to the first day of use the MYW ticket had.
The GR CMs have great latitude when it comes to handling ticketing situations so the above is not etched in stone. But I believe that you will find the far greater number of GR CMs doing it the new way.
Oh, and to answer the next question "What happens if the first use of the MYW was more than one year ago?" is that it will greatly depend on the CM. Some will offer you a dollar credit for the unused days towards the AP, others will tell you you can't upgrade it and have to use the remaining admissions as is. Untiil the policy has a longer track record, it will be hard to say which is the more generally accepted way.