CAWriter below response is not quite right..... The convertible seats ARE "real" booster seats. A booster seat is anything that uses the car's seatbelt to secure the child. When you start using a convertible seat as a booster, you stop securing the seat to the car and just use the seat belt to secure the child and the seat to the car. The purpose of a booster seat is to boost a child up to the right height so that the lap belt will fit across the hips and the top belt will fall over the shoulder blade instead of against the neck (ie the way a regular adult seatbelt is supposed to fit an adult).
The minimum for a booster seat should be 4 and 40. Most older carseats you had to remove the harness at 40 lbs. However, in the last several years there are a lot of new seats on the market that can harness up to 65 or 80 lbs and then convert to a booster. A 5 pt harness is always safer and is the best bet. Regardless of it's convenience or comfort. And most of the extended harness seats these days are pretty plush.
Your child does sound too big for his roundabout. But I would not be moving him out of a harness. I would look for a seat that has the higher weight limit harness. If you think this is silly or want the convenience of a booster, please take a look at these videos:
The first one by itself just shows you the dramatic difference between a child being securing in a 5 point harness and a child secured with an adult seatbelt in a bootser seat.
There are alot of seats that harness above 40 now......Graco MyRide 65 (which you can rearface to 40, FF and harness to 65 but does not convert to a booster), Graco Nautilus which is FF only but harnesses to a higher weight limit and then converts to a high backed booster, Britax has several good ones (I think Frontier is the only one that converts to a booster, and prob at this point I wouldn't buy one that didn't convert). It is a great seat, but it is very very big and can be difficult to install. There is also the Apex 65 and I think a couple Evenflo seats. You really have to go to the store with your child and read the fine print to see what is appropriate, try them out with your child and find one that is appropriate and then try it in your car as well. You want something with a good tight fit if you are going to use it as a 5 point harness seat. Toys r us will let you do this if you leave your driver's license....even if you dont' end up buying it there (they do run pricey).
When you do go to a regular booster, high back is better because it positions the seat belt properly over the shoulder. No back boosters usually do come with a positioning clip but most people don't know what it is and don't use it. High back boosters also provide some head support...sometimes side impact protection. If you have a car with a low seat back (no head rest and the child's head comes above the back of the seat) then you def should get a high back. I am not sure why anyone uses no back boosters unless you are buying one for a significantly older child who isn't quite ready to ride without one.
Here are the guidelines to know when you are ready to do without a booster: http://www.carseat.org/Boosters/630.htm
Most states the absolute minimum is 8 yrs and 80 lbs. But I would want my child to pass the above rules before I moved him out.
I had my older son who is tall and thin for his age in a harness until about the middle of his 5th year. I could have kept him in longer but I needed the seat for my younger child who was moving out of his infant seat. At that point I felt he was mature enough to sit properly in a booster (ie not be wiggling out of the belt, bending over, pulling the belt behind him) and it fit him properly. Test out how well it works as a booster too...making sure it positions the belt over the hips and shoulder blade, not against the neck or belly because if it does this and you are in an accident, a lot of soft tissue damage will happen instead of spreading the force over the hard bony structures.
Any time you are ready to transition or have any questions about specific seats there are a lot of great people with a lot of great info on the forums at this site http://www.car-safety.org/.
Also other posters are correct about the bulky coat. I know Chicago winters are harsh, but it is better to either remove the coat and put it on backwards, take the arms off, buckle the belt and then replace the coat, or wear thin layers and a thicker fleece jacket on top. Land's end and LL bean make some nice ones that I think are 300 gram fleece that would keep kids pretty dang warm without causing the problems with carseats.