DS1 is almost 3 years old and about 30 lbs. We currently have him in a Britax Boulevard convertible carseat. His younger brother, now 6 months, will be growing out of his infant car seat very soon. So we are left with the choice of:

1) buying another convertible car seat
2) buying a harness to booster seat (so we can move DS1 to this and move DS2 into his old convertible)

I've been trying to find information about harness to booster seats but the only hard and fast rule I've found relates to just booster seats (where they have to be 4 years old and at least 40 lbs). I take it though that this rule is for the booster seats (without 5 pt harnesses).

So anyone with a harness to booster? If so, when you did buy one (how old/big was your child)?