They should remain in the "convertible" carseats until they are 40 lbs or their ears are above the top of the seat back. They will most likely grow out of the seats in height before weight, so you would want to move to a "harnessed booster" style seat to keep your kids in harnesses as long as possible. Each time you "graduate" a step on carseats--from rear-facing to forward facing, from harnessed seat to belt positioning booster--you lessen the amount of protection your children have, so there really shouldn't be a rush to move up the steps.
Please do keep in mind that most safety experts reccommend destroying carseats by no more than 10 years after their date of manufacture. I used to be a certified technician (my certification expired in October and I chose not to renew), and at carseat checks we would confiscate seats for age alone at 10 years old--and anything older than 5 years old we were seriously looking for another "excuse" to take the seat.
Also, on the "puffy coat" issue...PLEASE don't use puffy coats with carseats! They cause you to loosen up the harness straps, so they aren't offering as much protection to your kids. Take off the puffy coats, buckle your kids up, then put the coats on backwards over the harness. At the very least, unzip the coats, and buckle the harness under the coats, not over the coats.
Good luck!
Jenn (mom of Jessica age 9 in a belt positioning booster, Katie age 7 in a belt positioning booster (was in a harness until she was nearly 6--she is tiny!), Jason age 5 in a belt positioning booster (he was in a harness until he a bit after his 5th birthday) and Sean aged 3 in a convertible seat...)
P.S. Holly said "i was even told she had to stay rear facing up to 30 pounds" This isn't quite right. Most convertible seats now, and even some infant carriers, can rearface to 30 or 35 lbs. As I said before, rear-facing is safer than forward facing, so I encourage extended rear-facing. My kids all rear-faced until close to their 2nd birthdays--or even past her 2nd birthday in the case of my petite second daughter. But the *law* simply requires you to follow the instructions that come with the seat, and since many seats allow forward facing at 20 lbs, you don't have to wait to 30 lbs to forward face. You should wait at least until the first birthday (corrected age for significant premies) to allow for bone development. Many children are too tall to rear face well before reaching 30 lbs--for rearfacing the child's head must be at least 1" below the top of the carseat.