When I got the call on 10/19 to switch from 5 seat to 6 seat, the gentleman on the line was clear that the switch would be higher priority. He never guaranteed by end of year delivery, nor did he provide a delivery date range for my 5 seat - the only thing he indicated was that I am high on the list, but that 6 seats would get me the car sooner.
I think they are building a batch of 6 seaters - we can tell that a person who ordered an original 6 seater on April 2nd got delivery. We can also see that a 5 seater who made the reservation in May, never got the call to switch over.
So, Tesla had an idea of how many 6 seaters they want to make in that 1st batch - they then went down the roster until they ran out of people who wanted them (that is a combination of original 6 seaters plus 5/7 seater converts... But the list is finite - and seems to end around March / April orders.
IN MY OPINION: By the time the 1st batch is produced, Tesla hopes to have the missing pieces (assuming it is the bench seat). Subsequently, it is unclear what cars are going to be produced - it could be PLAIDS, or a variety of LR configurations.
My hope is the 1st batch will be completed soon... And that I would be getting my car in February...