I have the same issue. Two of us, OGG to DFW to XXX, both booked on miles back in the spring, with assigned seats. I am lifetime platinum and my spouse is gold. I retained my seat, and my spouse lost his. The seat map is totally full. No flights are for sale in F for several days on either side of our dates, neither miles nor dollars. I am guessing your flight is like ours at the end of February. All of the airlines are very full, and other options are very expensive in both miles and dollars, way more than we paid. Finding an alternative option is going to be tough. Clearly there is a problem here because few of the folks who booked these flights are going to be happy being downgraded or changing plans even if offered incentives, because it is not that easy to change the return date of a Maui vacation in that timeframe. A lot of folks in F do this trip with the same dates every year, and there is work, etc.
My thinking is that one of three things will happen. They downgrade one or both of us to Premium Economy if they have it, or to coach if they do not. But they are still selling PE so there is a good chance that will be sold out on the day of travel. In any case my understanding is that when requested they will refund the difference between the fare paid and the fare they would charge if purchasing PE or coach on the day of the flight. Whether miles or money the refund will be very little. Not acceptable because of the refund situation and the likelihood of getting a middle seat in coach. Next option is to transfer all parties to the Phoenix flight and connect onwards for there. There is plenty of space on these flights but the Phoenix flight is a narrow body so standard recliner seats and not at all the same thing as a lie flat (and they know it) but I doubt they will offer any refund. Last option, reroute through HNL which is the least bad option. Currently there are F seats available on most days the OGG flight is oversold. Of course, there is the possibility they will upgrade back to a 777 if things pick up but that is probably unlikely as it stands right now.
A few years back the same flight that I was booked on was oversold and I received a phone call a few days prior offering $500 cash and $700 in vouchers if I would change the flight by a couple of days. I did not take the offer but at least they were proactive and we got on the flight as planned. My guess is they will do the same thing here but I doubt they will have many takers unless the compensation is substantial, and I think the problem is way bigger than just a handful of oversold seats on a single day.
I also called but they had no interest in solving the problem, citing that the flight wasn’t until next year and people would misconnect and cancel and space would open up. Maybe true for a mainland flight but not for this flight, most are originating in OGG and have set these plans in stone some time ago.
If anyone has any ideas or thoughts on what will happen, I would love to hear them.