miles upgrade tips?

Heading to LostWages with Mrs OS and the younger next year using miles; booked back o' the bus both ways. I want to upgrade to Prem on the return leg but currently the 4 seats that VS have designated for miles SLF are already booked. More MAY become available closer to the time, although thii is apparently unlikely given that LAS flights are normally pretty full.
My return date is pretty much fixed, does anyone know any process secrets (if there are any) that can help me to upgrade without waiting till the last minute (and probably being disappointed)?

Hi Oliver,
You best option would be to check a week before your return departure. The VS revenue team assess loads a week before a scheduled departure of all flights - If a high number of seats are available they will release more reward fares to reduce the chances of seats staying empty. This would be your best opportunity to snare any possible reward seating that potentially becomes available. I would certainly try a week before rather than waiting until the day of departure.....

Good luck....[y]