"The supposed reason being that if you are not quick enough changing the code, the box automatically goes to standby."
Huh? No understand. I thought the problem was that if you are interrupted during squawk change, you may display an incorrect squawk. How does 'the box' know when it should 'automatically go to standby'?
We've had 'active' and 'stby' VHF NAVCOMs with changeover buttons for years - why don't many transponders do this?
In an aircraft I'm involved with, the centralised system has a transponder page. The new squawk is entered with the rotary knob and then transferred to the active window by pressing the line select key. None of this 'going to standby' business! And it is not possible to switch off the transponders either!
I cannot believe that any professional pilot would deliberately switch the transponder off and lie about his real flight level......