I would stick with what you have and like; if the SPDs don't cause any pain or discomfort on long rides you could do much worse. I switched from Look to SPD to Speedplaytrying to get rid of some knee pain over distance rides. The speedplays were the only ones with a completely free-float that allowed my lower leg some movement over long rides, which immediately stopped any discomfort.

In a nutshell, if the ones you have are working you might end up inducing discomfort by switching to another system. That, and depending on which pedal you go with, you might have to re-learn the whole entry/exit routine.