In my opinion, there is no way to just do a conversion from EB3 to EB2. It would require a fresh LC application. Or what one could do is get the LC, apply for 140, and once 140 is approved, move on to a different company or get a new LC and apply for 140 with the same company with EB2. Since the old 140 is already approved, you would be able to retain the original Priority date.
Unless one goes through this convoluted and time consuming process, no conversion is possible.