That's a tough one. Luckily, it does NOT happen to most kids! Usually when they get mobile (crawling, etc.) their gain slows down.
First, I'd see if I could get a doctor to prescribe a Swedish seat (one with a 55 lbs. rear-facing limit.) I would not care if I had to pay for it OOP, although I would get insurance to pay part if I could. With a prescription, it would be legal for me to use said seat for said child.
If that did not happen (but it would-- my doc is EXCELLENT about car safety) I would carefully debate whether I would prefer to go off-label on a Radian (because of many factors, that's the ONLY one I'd go off label on) and rear-face in it until the kid hit 40 lbs. or the first birthday, or whether to buy a Swedish seat and use it illegally. Both options are illegal; one has been crash tested and the other hasn't YET.