This is how the language reads in my paper 2011 Gold membership benefits guide (p. 5):

ROOM UPGRADE
Upon check-in, you can look forward to being upgraded to our best-available guest room. Upgraded accommodations are at no additional charge based on availability. Suites are excluded. Upgrades are not available at Marriott Vacation Club. Upgrades from studio to one- or two-bedrooms not guaranteed at Residence Inn and TownePlace Suites.

The new website language is definitely a purposeful change; there is no way it is a mere clarification as "best-available" is worlds apart from "next category"