VS5 LHR;MIA: 747 or Airbus346

Since the Airbus 340-600 has operated Virgins London to Miami route weve been especially delighted because Er Indoors gets very airsick, NOT through boozing, despite what fellow travellers and crew think. Its been manageable by getting seats allocated as far forward as possible in the Economy cabin. I admit were cheap, though if thered been a few more like us these past 10 years the world wouldnt be in this mess. But I digress. The 2-4-2 layout of the 346 suits us much better than the 3-4-3 of the 747. By booking early Ive always had a window seat and she has the aisle seat next to me because it helps her to drink lots of water, but that means many trips up & down to the rest room. As its not a 747, theres no third seat for some poor sod to be embarrassed by her antics, or disturb us further by his.
Just like the last dozen or so flights over the past couple of years, we had the usual excellent LHR to MIA flight back in mid-November, having been allocated good seats, both outbound and back next 7 May. One thing was different: the pleasantness and helpfulness of the immigration guys at MIA, non-Americans being urged to use the much shorter US Passports Only lines. A welcome and long overdue change.
Just recently I checked our reservation online and was disappointed to find theres been a change of equipment: now, a 747 is to fly us home, so weve been allocated new seats, well down the back, and not now changeable, even though were not travelling for 7 weeks.
I understand perfectly that neither aircraft type nor seats can be guaranteed, but does anyone know
1. If the change of aircraft is a one-off and were just unlucky, or is the 747 going to be a regular feature of VS5?
2. Is it better to try to get the Memsaab moved forward to an aisle seat now, or online in the 24 to 2 hour period before departure, or on boarding the aircraft?
Just like the last dozen or so flights over the past couple of years, we had the usual excellent LHR to MIA flight back in mid-November, having been allocated good seats, both outbound and back next 7 May. One thing was different: the pleasantness and helpfulness of the immigration guys at MIA, non-Americans being urged to use the much shorter US Passports Only lines. A welcome and long overdue change.
Just recently I checked our reservation online and was disappointed to find theres been a change of equipment: now, a 747 is to fly us home, so weve been allocated new seats, well down the back, and not now changeable, even though were not travelling for 7 weeks.
I understand perfectly that neither aircraft type nor seats can be guaranteed, but does anyone know
1. If the change of aircraft is a one-off and were just unlucky, or is the 747 going to be a regular feature of VS5?
2. Is it better to try to get the Memsaab moved forward to an aisle seat now, or online in the 24 to 2 hour period before departure, or on boarding the aircraft?