I think you'll find that operators don't get a choice. The clue is in the word "rules".
Actually I know one operator of the QF group that does not use the new rules. Hence why I said this. They claimed the new fuel and alternate requirements were uneconomical so stuck with the old requirements, including use of special alternate minima and such (which part 121 can not use anymore). That may have changed in the last few months, but was like that for some time after the new rules were adopted.