So let’s say I have an AP that’s expiring in 6 months. I then buy a one year license and place it on that AP to extend the license. What happens if I buy a new AP? Is the partial and new license gone? Do I have to buy a new license all together?
In the traditional co-termination model, if you had already used 1 AP license for 6 months (and had 6 months remaining) and then added another 1-year license, using the "renew my license" operation, you would now have 18 months from today. If you added a 2nd AP and used the "add more devices" operation, you would have 6 months of unused license on the first AP, 12 months on the new AP, therefore 18 "license months" divided by 2 APs, and your new co-term date would be 9 months from today.
Now in the per-device licensing model, you have the option to have each AP licensed separately (if that's what you want) and your first AP's license would still expire in 6 months and the 2nd AP in 12 months. But you would also have the flexibility to arrange a shared expiration date. This is covered in the section on shared expiration dates and leverages the 1-day license SKUs. So you have the flexibility to buy 182 1-day AP licenses (6 months) for the first AP, along with a 1-year license on the new AP, and that would then give you another full year for both APs.
What you can NOT do in the PDL model is split licensing across multiple devices. That 1-year license on the new AP needs to be applied in full to an AP, it cannot be split across both APs. The support doc mentions this as well, it explains how, if you have 12 APs, you cannot buy a 1 year license for the purpose of adding 1 month to 12 devices.
Look over the doc and let us know if something's still now clear. Hope that helps!