Never tried to do that - had some fun changing to Velocity at lounge/gate (on flights for which R upgrades had been applied).
With that it seems they can do for individual legs, as after changing my outbound flights, the return flights stay under Airpoints.
http://www.flyertalk.com/forum/air-n...e-earn-ua.html
http://www.flyertalk.com/forum/air-n...ding-pass.html