How much time must pass once an annual pass expires before a new annual pass could be purchased and not have its activiation date revert to the end of the previously held annual pass?
From what I understand the new AP will only have an activation date of the end date of your old AP if you renew the AP rather than buy a new one. The renewal price is cheaper than buying a new one.
When you buy the actual AP (like when you purchase it at the gate), I believe the activation date will be the date that you purchase it. However, if you get a voucher (like you would get at The Disney Store) then the activation date would be the date that you exchange it at the gate for the actual AP.