It was mentioned in the Spring 2008 Disney Files magazine. It probably changed in October 2007 since that is when the Carolwood Pacific Historical Society generally meets at Disney World.
On our last trip my son mentioned MGM and I said "it is actually called Disney's Hollywood Studios now". He said "well I call it MGM" Guess it is hard to change something you have known forever.
The date the name changed was mentioned during the 4/7/2010 Dis Unplugged podcast. A listener tried to stump the Dis Unplugged crew by asking the name of this room during "Stump the Roundtable" on 3/31/2010. The Dis Unplugged crew researched the issue and found the answer.