We did this one time. If I remember correctly, the Guest Services window was open about an hour before the park. At least we arrived about an hour before. But I do know we were one of the first in line that morning....both at the guest services window and at the turnstile.
Be sure to check your receipt closely. One time, we converted unused park hoppers to APs and somehow the amounts were screwed up, to the tune of almost $100. We noticed later in the day and it cost a bit of extra time to get the problem straightened out at City Hall.
HTH!