Glad to hear it worked out.
Technically, Dollar has a nationwide policy of holding reservations for 2 hours before no-showing/cancelling them at suburban locations...and for 6 hours at airport locations.
Any variation in this policy should be noted in the reservation terms and conditions.
Still, if you had not been able to change your flights and you had not wanted to chance it, you should have been able to call a Dollar reservations sales agent (800-800-4000) and have them manually override the rate change when changing the pickup time within the same day (which is also official policy). Now that the callcenter has been outsourced to Jakarta, though, you may not find that avenue successful, though it might be worth asking to speak to a supervisor.
The third way of doing this is to try to contact the location directly, since they can modify the pickup time in their system (which is not tied into the reservations system and therefore should not trigger a rerate). Unfortunately, it is virtually impossible to find the phone number for corporate-owned stores (which I believe OGG is), since Dollar seems to like to dump all calls to the cheaper outsourced callcenter rather than staffing up individual locations to take phone calls (in phone books and on the website, they usually just publish the main 800 number). Still, with some Google sleuthing, it is often possible to locate the direct number (also look in your reservation's terms and conditions for a local number to call for shuttle pickup--that will often be the way to reach the counter staff directly).
As for the 6-hour no-show policy, I'm not sure that it's in writing on the web, so I'd be hesitant to rely on it unless you can find written proof.