VS46 Aircraft type

I am due to go out to JFK week after next returning on Sunday on the VS46. Outgound is scheduled to be an A346 on the VS9 and return on VS46 on the Sunday a 744.
For the colleague I will he travelling with this will be his first trips on widebodied aircraft so good he gets to experience both. However I see some substitutions of VS46 to be an A346, such as today. I assumed that this substitution would happen depending on loads. However I see from Experflyer that today's VS46 looked almost full on the scheduled 744, so is likely to have to offload passengers now it's a 346.
The outward and return VS045/VS046 also look pretty full.
Is there a shortage of 744's at the moment? I woudl have thought due to the loads on these flights that VS would want to keep the 744 operating.
I see Mustang Sally is currently en-route to TPA on flight VS965, and has a slot already in it's schedule to return as VS966 on the 24th
For the colleague I will he travelling with this will be his first trips on widebodied aircraft so good he gets to experience both. However I see some substitutions of VS46 to be an A346, such as today. I assumed that this substitution would happen depending on loads. However I see from Experflyer that today's VS46 looked almost full on the scheduled 744, so is likely to have to offload passengers now it's a 346.
The outward and return VS045/VS046 also look pretty full.
Is there a shortage of 744's at the moment? I woudl have thought due to the loads on these flights that VS would want to keep the 744 operating.
I see Mustang Sally is currently en-route to TPA on flight VS965, and has a slot already in it's schedule to return as VS966 on the 24th