If you are just putting trains together and running them around the layout, I'd leave your old cars as is (at least for now). If you are planning on doing operations (switching, ect.) then you really need to switch over to one of the knuckle couplers. Since most new equipment will have knuckle type couplers, you'll need to build a few "adapter cars", which is a car with a Rapido on one end and a knuckle coupler on the other. Again if you are just running trains, you can buy unimate couplers, which must be coupled manually, but that doesn't matter if you are just assembling trains and letting them run. The advantage of the unimates is cost.......you can usually get enough for 4 cars for about $5 or less. The other advantage is the unimates are a direct replacement for Rapido couplers....they snap right into the same mount.
At some point you'll want to switch over to knuckles, but unimates and adapter cars will get you up and running for the least money. When you start swapping to knuckles, Atlas Accumates will usually be the cheapest......the main problem I've had with those is the metal pin on the couplers usually disappears (falls out) which stops them from uncoupling magnetically, but doesn't effect them otherwise. Micro-trains are what most people prefer, and are always a good choice, but also usually the most costly. McHenrys seem to work well, and fall in the middle price-wise, but are also a little more delicate. Overall, they all work and they work with each other, so go with what you can get cheapest.