More plausible than you might think.
OK, first have the Austrians not get their butts kicked in the Netherlands and Italy. On the Rhine front, the Austrians overrun French Lorraine and Normandy in a counterattacker after a successful defense of the Netherlands (defeated armies are always easy to chase away). The Pragmatic Allies now have a lot of maneuvering room in captured territory. The British can send another 3,000 troops from the European theater to the New World, where they were already winning at Lousiville and had threatened the Ohio area. This little "boost" will give them what they need to take land, and with Austrians and Savoys stalemating the Spanish and the French having an epic fail, they can't trade European land for their colonies back.
OK, minor problem. I think the Duke of Marlborough, one of GB's best commanders, is dead. And Austria had an epic fail on leadership, at least he British had good ones but not as good as Maréchal de Saxe.