If you want to go back to ACR don't omen the image into Photoshop, Instead hold down the shift key or set you ACR work-flow option to Open the Image as a smart object layer. When your in Photoshop you can double on the layers smart object icon in the layers palette to reopen the conversion in ACR and change settings.
That will definitely work and thank you for the reply. There is also a command shift "something" that enables you to toggle back and forth. I found it a few weeks ago, but now it escapes my memory.
If you are running PS-CC, there is a new Camera Raw filter that lets you use the camera-raw functions on any layer you want. This is available from the Filters menu.