Its really hard for my older son to get into the back with two rfing carseats in the middle captain's chairs as well.
He climbs through the trunk.
I have stow n go seating, so i keep 2/3 of the rear seat folded down so our double stroller fits back there and my son just climbs through. I dont know what i'd do if i had to have another carseat back there. The only thing that is a possibility is to make sure the middle seats are pushed all the way back which leaves a little more room in front of them to be able to climb through. Also , sometimes my son will get in the passenger seat and then climb all the way down the middle to the back but thats a hassle too.
When i had my little toyota corolla i thought minivans were so huge...they hold SEVEN people after all. Now that i have one, its hard to imagine FIVE kids in the back, esp with several carseats. My friend may be adopting a sib group of three kids in addition to her two, which means she would need at least two convertibles and three boosters (depending on the size of the kids, she'll have a 3yo, 4yo, two 5yo, and an 8 yo)...i cant imagine how much that will suck trying to get everyone in and out several times a day.
For the original question...i think if my son was at least three and outgrew his Blvd i would be ok switching him at that point but i'll cross that bridge when we come to it...who knows what seats will be out in a year?