Hi,
At one point Transport Canada defined a child as someone under 48lbs and as such our car seats were capped at that limit. Around 2006 (?) the definition of a child changed to 65lbs, hence your newer Marathon allowing for use up to 65lbs similar to it's American counterparts.
All that to say, it is
not okay for you to use your 48lb seat past 48lbs. No one is sure if there were structural changes made to suddenly allow our Britax seats to pass up to 65lbs, and it would be illegal for you to use your seat past it's stated weight limit -- especially if your province has a proper use clause.
Keep in mind that very few children last in a Marathon to 65lbs anyway. A Marathon is outgrown when one of the following this happens:
The child reaches the maximum weight limit
OR
The child's shoulders are level with the top harness slots
OR
The tips of the child's ears are level with the top of the seat;
Whichever comes first.
Most children outgrow a Marathon because their shoulders are level with the top harness slots, and that happens loooooong before 65lbs is reached.
Hope that helps,
