I recently come up with that code too, I hope its not the O2's and just the cats not working too well. I have recently change to a Cat back exhaust system and done some other engine mods, tunes a little using HP tuners. Sometime when shifting from first to second at 4500rpm I hear a pop from the exhaust. Decelerting down a dark country lane in second gear about 2000rpm, I see flashes behind me. Appears fuel is igniting in the exhaust system which probably wouldn't do the cats or O2's much good. Think i need ot tune that out, it dosn't happen all the time so may be hard to find.
I have never looked at swapping the rear O2's, are they on a plugable harness? my car done 120,000 miles so I guess they will be pretty much welded in there and be a bugger to remove.