It has happened from the 738 to the 744 (FO to FO, 744 FO to 737 Capt, Capt to Catpt) I haven't known of any expat moving from the Boeing fleet to the Airbus fleet or vice versa.
Any expat who joins CAL should know that they are requiered but not welcome and for that reason you are going to be the last to be consider for an upgrade or a fleet transfer.
It is not a long term job not even to consider an upgrade or fleet transfer.