There's no command for switching between the two.
I suggest using a limbo-screen kind of thing, perhaps with dialogs.
A trigger, which has cadetMode in it's condition field, and has [] exec {cadetModeOn.sqs} in the onActivation field.
cadetModeOn.sqs: (non-dialog version)
Hint ["I'm sorry, but you are running the game in Cadet Mode, please switch to veteran mode and restart the mission."]
~10
forceEnd
Then you could do a fancy version with dialogs.
Create a simple dialog (tutorials are available) and give it the name DlgCadetmode.
My suggestion is popping a little box up, with some text (the one in the hint box above) and an "Ok" button.
For the "Ok" button, add this action in the description.ext:
action = "forceEnd";
That way, the game will end when the player clicks the "ok" button.
Initialize the dialog in the cadetModeOn.sqs script by adding this:
_ok = createDialog "DlgCadetmode"
?(not _ok) : exit
at the beginning.