744 or 343

Of the last six VS011 LHR - BOS flights, 5 have been shown as a 744. The Virgin Atlantic site for this flight on 23 May shows the aircraft to be a 343. What are the chances of:
a. the flight actually being a 744 and
b. being able to choose upstairs PE seats for our flight[?]
I do appreciate that there is still a fair way to go until the trip but I'm sure some of the experts here will have some input:D
a. the flight actually being a 744 and
b. being able to choose upstairs PE seats for our flight[?]
I do appreciate that there is still a fair way to go until the trip but I'm sure some of the experts here will have some input:D