In Argentina we make the change during climb ONLY after crossing the TA (even if we were cleared up to a FL).
Its clearly stated that it should be done only in this fashion in the AIP under ENR 1.7 (spanish only).
The only exception is when you are cleared for an approach. In this case, you can make the change from QNE to QNH while above the transition level (instead of after leaving it) if you were not instructed to maintain a level above the TA.