Change of aircraft

Could anyone tell me from they're experience what happens if a change of aircraft means that seats are no longer availible in the class booked, for example PE on a 747 but the aircraft is changed to a A340 that has a smaller PE section, would you be bumped up to UC if there was room or offered miles and economy etc as compensation? thanks

[preiffer]

You'd be more likely bumped to Economy, as there would be less J seats available as a result also.

Having said that, they'll usually swap aircraft around capacity, so in most cases they would have made sure all the passengers 'fit' into the new plane config where possible. IF it did happen, downgrades and compensation are far more likely than upgrades I'm afraid.