There are a few options that occur to me. One would be to get an acoustically transparent screen (assuming that's an option). Another would be to relocate the original center to below the level of the screen (which might also involve lowering the plasma a bit so the bottom edge of both the plasma and the projection screen line up). A third would be to put a selector switch at the amp output to select between the two centers. There would be the potential for level calibration issues with that third option (although not necessarily if you had identical center speakers); if there was one, you might be able to put a gain control in the louder signal path to pull it down to the same level as the quieter signal path.