By the time you have manouvered away from the treat and it away from you, if you return to the original level the threat would have gone. TA is approx 40sec and RA approx 20 secs from CPA...the manoever and return takes far longer than the 20 secs. If you dont go back to the old level you could be setting yourself up for a further conflict at the new level your at....especialy in RVSM airspace with 1000ft clearance between opposing levels.