I don't quite understand what the issue is.
Is there availability for the new itinerary (new destination and your chosen date)?
If so, the UA website allows you to change to the new destination (and date change also, if that's what you want)
For example, here's one of my award itineraries:


By choosing the "Edit" option, I can change the departure airport, destination airport, and/or travel date.
If there is availability, I can rebook to the new itinerary.
In the example below I have chosen to change all three search parameters, and the new itinerary is available for booking and ticketing.


But I could also have chosen to keep the departure airport and date and only changed the destination.

Can't you simply change the destination and reissue the ticket on the UA website?

As for award seat availability and if/when seats are available if you cancel the current itinerary, the answer is most likely "nobody knows for sure".
Perhaps the seats will become available, perhaps they won't.