I concur with using either Kadee or Walthers.
If you have the cheaper plastic ones, it's ok to leave them in place until they fail. All knuckle couplers are compatible, just differ in quality. But the so-called hook-horn couplers don't work with the knuckle couplers and will need to be replaced.
One technique you can use, if you don't have the time or budget to replace them all at once is to make a transition car with a hook-horn at one end and a knuckle coupler at the other. This way you can run both types in the same train.
Connecticut Valley Railroad -- A Branch of the New York, New Haven, And Hartford
"We don't stop playing because we grow old. We grow old because we stop playing." --George Bernard Shaw