At once and Until now

At once adverb - Without delay or hesitation; with no time intervening.

Until now and at once are semantically related. In some cases you can use "Until now" instead an adverb phrase "At once".

Until now adverb - Used in negative statement to describe a situation that has existed up to this point or up to the present time.

At once and until now are semantically related. Sometimes you can use "At once" instead an adverb phrase "Until now".