It all depends on the transition altitude on MCDU take off page.
In your example , MCDU transition altitude is 6000 ft. All constraints below 6000 ft will be altitude , regardless of your baro QNH/ STD setting. Early selection of STD will not affect the constraint and AP/FD will give guidance to maintain the barometric corrected FL.
So if QNH is 1003 , with 300 ft difference from STD , the AP/FD will guide the aircraft to level off at FL37 with FL37 magenta on PFD.
Opposite in descent , if TL is 45 on MCDU PERF APR page , and on FLPLN there is a FL constraint of let's say FL 80.
The FMGS will understand it as Flight level as it's above Transition Level inserted on MCDU and it will correct the barometric difference.
So early selection of QNH, will not affect the maintained FL constraint and with 300 ft difference the AP/FD will maintain 8300 ft , PFD will display FL83 magenta. Safe side!
The governing factor is the transition altitude inserted on MCDU
It goes without saying that this is not valid for OPEN modes
Last edited by Citation2; 24th Dec 2012 at 23:19.