Mike can certainly correct me, but I think what he meant, as Kevin also mentions, is that the opt-out/waiver does NOT have to be irrevocable (unless of course that's all the document permits)
If it isn't irrevocable, then it may be a deemed CODA, which has various implications for testing/limitations. Like Kevin, I haven't seen a pre-approved document (which is mostly what I see these days) for a long time that allows anything other than the irrevocable waiver/election.