You may find in the long run that it's better just to a) scrap the truck-mounted couplers and mount them on the body, or b) just scrap the cars as you get newer ones.
The first reason is functional - body-mounted couplers cause fewer derailments, especially if you are backing a train through curves or switches. When you are backing a train with truck-mounted couplers, there's a side-force on the coupler that can cause the truck to skew and de-rail. With a body-mounted coupler, there's no force on the trucks, and they'll track properly.
The second reason is both functional and aesthetic. Firstly, if you have a car with truck-mounted couplers, chances are that the truck is mounted to the body with a split barbed pin. They are loose, and let the car wobble around on the trucks. This can lead to derailments. Secondly, these trucks tend to have cheap, 1-piece injection-moulded wheelsets. These typically have large flanges, and are sometimes out-of-round. Better cars will have the couplers mounted on the body and the trucks (with plastic wheels on metal axles) mounted with screws. In short, they'll run better and more reliably.
Also, the cheaper cars (with the truck-mounted couplers) tend to be poorly painted, with thickly applied paint and/or poor graphics. As you get more, better looking cars, you'll find the old cars really look cheezy next to the newer ones.