I expect some of the 789s will be integrated into existing schedules, allowing for deferred maintenance of some overworked planes. There may be up gauging of airframes on certain routes, but AC bean counters will balance that against the current high yields they're getting. Some of the 789s may indeed be subject to maintenance themselves, and/or wifi installation, and/or repainting to the 'new' livery'. It's a fair bet that few, if any, of them will be 'sitting dormant' while China routes are suspended.
One important item to remember: none of the individual planes in AC's fleet are assigned to a specific route. Any of the Dreamliners could be used to fly to Asia then Australia then Europe and within North America in any given week.