Changing codes
With the rotary selectors, you need to select standby first. ATC will expect this and know you may fade for a few paints, however, your label may not disappear entirely if the radar has a "coast" function or there is also primary radar coverage. The problem with not selecting standby is that in busy airspace, as you scroll through the digits, you may scroll through the code assigned to another aircraft which will give ATC a dupe. I.e. two different targets with the same callsigns.