Here are the specs from Britax Marathon70 (if you have a different model, lemme know).
Rear-Facing 5 – 40 lbs, a standing height of 49 inches or less and seated shoulder height of 9 – 16.75 inches.
Forward-Facing 1 year & 20, up to 70 lbs, a standing height of 49 inches or less and a seat shoulder height of 12 – 16.75 inches.
So the seat is safe rear facing up to 40lbs *as long as* shoulder height is no more than 16.75 inches. It's just safe in the seat FF above 20lbs.
The 2 most important things are not exceeding (or being under) the weight for facing one direction or the other AND that their shoulders are UNDER the the hole for the straps to go into.
So you're good for another 18lbs (not sure how tall your wee one is yet).
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My son was over 20lbs at 6mo, and we had to buy a whole new car seat at the same time (shoulders were over the top hole, different brand, but by age 2 he was 37.5 inches). We had to get a "common sense exception" from the fire department and pin it to his seat. If we had been in an accident with him over the weight for a seat both at 6mo when we went FF and again at age 1ish when we had to switch to a booster. In both cases, it was because he exceeded the weight and height for the seats we had (10 years ago we had the biggest seat on the market. Because he exceeded the weight the seat *could* have failed... and with his shoulders OVER the hole (the most important bit) it *would* have snapped his spine from the pressure of the straps.
But ypu're good to go. You're WELL within the safety margins. 20lbs is just the MIN to turn FF. And remeber some 2yos, and even 3yos aren't 22lbs yet. You CAN go FF, but don't have to for another 18lbs.