Originally posted by SnackyStacky
What you're talking about is an upgrade, and it can absolutely be done!
Just go to any guest services window (except at the resorts - they can't do it there) and tell them you'd like to upgrade the days remaining on your park hopper to an annual pass.
I know this may seem trivial but it could greatly affect your ability to do this transaction. In Disneyspeak (which is quite different from English), an upgrade can only be done within 7 days of first use of the ticket. You may very well take that ticket to a Guest Relations window, ask to upgrade it and be told you cannot. It has happened more than once and several times that I've heard of lately.
What you should be asking for is to apply the dollar credit remaining on the ticket towards the price of the AP. This ttransaction can always be done no matter how old the ticket is.