If they're still going to bother to assess the content post-release, what is the point of changing the system? It just prevents them from catching anything unexpected before the damage is done. The Times article mentioned there will be "penalties for nondisclosure". That's well and good, but it's the industry as a whole that suffers if the ESRB misleads parents and it becomes a scandal that prompts more restrictive legislation.
I suspect that the "post release testing" will be very minor if it's performed at all. After all, the public will be able to test the game at that stage. Not really testing post release the only way to explain exactly how the ESRB is saving money with this move.