Law 21 B 1 (a) states:
Until the end of the auction period and provided that his partner has not subsequently called, a player may change a call without other rectification for his side WHEN THE DIRECTOR JUDGES THAT THE DECISION TO MAKE THE CALL COULD WELL HAVE BEEN INFLUENCED BY MISINFORMATION GIVEN TO THE PLAYER BY AN OPPONENT......’
How is the Director to make this judgement, other than by simply asking the player concerned? You can take him away from the table and ask him whether he wishes to change his call and if he says yes, then should you question him further? How can you make this judgement without looking at the player’s hand, which you are not supposed to do unless as a last resort?
I can imagine that a TD would be very very reluctant to deny the player the opportunity to change his (last) call.