Chad,
I would recommend that you don't change the cars from your "starter" set. I still have mine from when I was 7, and I display them on a shelf above my workbench.
The reason is that changing the couplers is just the beginning. Those old cars tend not to be very accurate or "prototypical". To get them to run really well, you'll likely have to change the couplers, install metal wheels, add more weight, and then you might want to change some of the details like moulded on ladders, add weathering, possibly repaint or redecal, etc, etc.
You are probably better starting with a new "fleet" of inexpensive cars from Accurail (yellow box) or Athearn (blue box). They work well, and only take a few minutes to about an hour or so to assemble. They often have "knuckle" type couplers, and the couplers are "body mounted".
Keep your old train for the memories, but run the newer stuff.
Andrew