The current lawsuit states the mandate is unconstitutional because the requirement to purchase medical coverage has been removed. ACA was changed, so whether or not the individual mandate is still constitutional will have to be tried, likely bumping up to the SC.
Actually the ACA was not changed.
It was the tax bill where they changed the individual mandate tax to $0.
That is what made state attorneys general in conservative states file this suit and a Texas district court judge ruled all of the ACA unconstitutional, a decision that most people expect won't stand in appeal.
But the DOJ on Monday wrote a letter to the Appeals Court considering the case that the lower court ruling should be upheld in its entirety.
They are expected to submit a detailed brief later.
Meanwhile there's an article that two cabinet members, Alex Azar of HHS and Bob Barr of DOJ both opposed the administration taking this position, to have all of the ACA ruled unconstitutional.
Azar was against it because they had no replacement policy.
Barr was dubious of the case and the ruling and don't think the Appeals Court will uphold the lower court ruling.
But the White House overruled them and went with the push by Mick Mulvaney, who has a long history of opposing the ACA. For instance, when he was in Congress, he supported a govt. shutdown to try to stop the ACA.