Call Marriott Rewards up. They can't "change" your promotion such that you will automatically get more points, but they can put a note in your account that you preferred a different promotion (i.e. 20 nights/35K points). That way, once the promotion is over, you can call back and they will add the extra points to your account.

I am doing that for the first time with this promotion, but based on other posters in previous Megabonus threads, there haven't been too many issues with getting this done.