marathon height questions

My 3 year old (turned 3 in April) is tall and long in the torso. I don't know his exact measurements now, but he was about 39 inches in April. He outgrew his Roundabout, and is now in a Marathon in my car and a Regent in DH's car. We recently had our carseats checked and they said that in forward facing seats, the shoulders should not be above the height of the tallest slots. I checked the Marathon manual and it says that the carseat may be used after that happens, until the tops of the ears are higher than the seat. According to the car seat techs, in an accident he would move forward too much if the straps are coming from lower than the shoulders. Would someone please clarify? I was hoping to use the Marathon another year or so, at which point we'd get a Parkway or Compass.
thanks
Jennifer

I think the manual is saying, 'warning... this is your last warning... this kid is outgrowing this seat, seriously...' but in not so many words (I've seen kids in other brands of seats totally way too tall, heads sticking all the way out, Britax is really trying to avoid that issue).
So yeah, basically, you don't want the shoulders to go very far above the slots...a teeny smidgen is ok, and that's when the tops of the ears would be even with the top, anyway.
Where is your ds now in the Marathon? If he has like an inch till he's even with the top slot, it WILL be another year or so till he's above the top slot...kids put on a lot of leg at this point, and the Marathon fits till most kids are wearing at least size 6 shirts. Here's a pictorial (click the pictures for more detail) of lots of kids in MA's and Huskies/Regents so you get an idea how much longer you have http://www.cpsafety.com/articles/FFalbum.aspx
Julie CPS Tech and mom to 3 in seats
http://www.cpsafety.com/articles/RFAlbum/SarahMA.aspx