Then and Until now

Then adverb - In addition to what has been said.

Until now and then are semantically related In some cases you can use "Until now" instead an adverb "Then".

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

Then and until now are semantically related in earlier topic. Sometimes you can use "Then" instead an adverb phrase "Until now".