There are a few "one time exceptions" that are tracked, and therefore strict. Status matches are the canonical example: usually status matches are "Once per lifetime of account" or "Only once per 5 years," and they track these things.
In general, though, getting a "one time exception" means "I'll do it this time, but it's not policy (or against policy) so don't count on it next time, and don't cite this as precedent."
Which means that if you find yourself in this situation again, you're free to ask for the same exception, and you might get it. But it's against policy, and citing this experience as precedent will get you nowhere.
^
So you explained to the agent you were doing a MR, suggested SFO, and she accommodated you to there instead of SAN? That's a (cough PMUA) nice agent.