What could happen if an infant exceeds the weight max for a rear facing infant carrier car seat?

My baby is 8 mos old and weighs 20 lbs. The car seat that he is currently in is a rear facing infant only carrier/car seat combo with a weight max of 22 lbs. Obviously he is to young to be turned to a forward facing car seat. My husband has an extra car seat in his car for our 2 year old to ride in his car that is a convertible seat. I want to move it to my car for the baby (rear facing of course). But then our 2 year old wouldn't be able to ride in "Daddy's truck" (xterra) without us having to move car seats around. Not to mention, my husband wants to wait a little longer before we move the seat to my car. I'm afraid we may be putting our baby in jeopardy because he is approaching the car seat max, he says he'll be fine (the baby that is). My husband wants to wait a few more weeks. So this leads me to my question. What could happen if an infant exceeds the weight max on a car seat? Am I being a little ridiculous here?