Same-day confirmed change would work in this case, but the choices are limited. The OP didn't post details but let's assume the original flight is scheduled to depart @ 7:05 PM Friday night. The "same-day" change cannot be confirmed until 7:06 PM Thursday night and only for flights departing between 7:06 PM Thu. and 7:06 AM Friday. This leaves the OP with only 1 departure option Thu. night - the 8:45 PM. This would have to be done at the airport since it only leaves a 1 hour 45 min window between the SDC and the departure time.

Right now it looks like a good possibility since all Thu. PM flights show availability in all fare buckets.

As for the steep add-collect, it does sound like the whole ticket was re-priced since the OP purchased this ticket months ago and probably had a 14 or 21 day advance fare for which he s/he no longer qualifies.

...Oh, and JD3030 - welcome to FlyerTalk!