seems that no two operate the same. Some go to standby when you change one digit, some transmit the new code as soon as a digit is changed, and a couple transmit the new code two or more seconds after the code has been changed.
Hence my position about selecting STBY before changing. I wouldn't have a clue about the TXDR's behaviour while changing code. There's certainly no information supplied in any documentation I've seen for any of the a/c I've flown. Maybe in the engineering documentation...