If I am reading you correctly you already have a waiver in hand for migraines. Traditionally unless the issue changes, i.e. now need meds that aren't allowed or your health creates a deployment issue, they will allow that waiver to remain in place.
However, for you that is the key, if this is the 1st time you are seeing a neurologist they may discover that is not migraines, but some other issue (not saying tumor or anything dark). Under that scenario you would need a new waiver, and that is when it becomes iffy.
I am also assuming that you have not done your out process DoDMERB. I know for rated it is a 3 day physical. I don't know what non-rated does or when it is performed, guessing very soon, but remember assignments only came out on Thursday, so no non-rated has yet to go through their physical. This physical would have triggered you anyway regardless if you had reported the migraines...yes, you were 100000% right to report it. That is what any service member should do, because your health impacts everyone in your AD unit to complete their mission.
Are you going to a DodMERB qualified doc or just your hometown doc for the neurology appointment. I would suggest you try to go to a DoDMERB because as you know by now, just because your hometown doc says it is n't a problem does not equate to the military saying it won't.
Hope for the best, but for right now get your medical records in order to fight in case you get the worst.
The det. can't do anything right now because the paperwork has to go up the chain and back down the chain. You have to see the doc 1st. and they have to make a decision. Just like your waiver yrs ago it may take a few weeks...or months.
Good luck.