No, you will need to reflash the PRL back to Sprint. I have a Moment on Cricket, and after using the Sprint tool to update the Moment to Android 2.1, my service on Cricket still works just fine, with no modification.
So, I believe that if you want it to be on Sprint's network again, you will need to reflash a Sprint PRL. You might be able to do that through the update PRL command in settings, howver I haven't tried that, as I want it to stay on Cricket service.