No - only platinum members can change their promotion at will (well, by sending an email to the concierge or calling up the plat desk). Even then you then don't really change promotion they just manually add the additional points at the end of the offer period (or when you hit the target). This is just a 'special' case to reward their best customers and until I started reading FT I never realised it was possible

Of course, you could always try and ring up and ask if you have a good reason... don't think it would work though as they are trying to use the promotions to change people's staying habits (i.e. stay more at Marriott!!) to benefit their bottom line, not to give away points for no reason!

Edit: Looks like I was mistaken, based on other posts below - seems anyone (??) can give it a whirl and ask for a promo change, so, good luck if you try