A few things to help you determine if you can replace just the door panels:
What is the thickness of the existing door compared to the door you want to purchase? If your existing door is the older wood style, they typically are 1-3/8" thick. The new steel doors vary in thickness, from 1" to 2". Your track spacing is based on your door thickness. Using your track with a 2" thick door may require spacers behind the track mounting brackets to make up the difference.
Depending on the weight of your existing door, and the weight of the new door, your existing springs may be too strong, or too weak. Springs are calibrated for the specific weight of a particular door. You may have a problem balancing your new door if you use the old springs.
Age of the remaining components. You don't want to start getting nickel and dimed to death having to replace the hinges and rollers, which can get to be expensive buying them individually. If the roller wheel on the shaft wobbles, the ball bearing are shot, and it should be replaced.
The big box stores only stock standard 8x7, 9x7, and ocasionally 16x7. Any other size and you will pay a premium. You may want to consider getting a cash and carry price from your local dealer. They will usually give you a better installed price as well.