1) Fire everybody
2) Create new corporate entity
3) Rehire the meek and compliant
4) Sell corporate entity back to original company
Judges tend to get very cranky when they have to deal with shenanigans like these.
The question really should just be in #1 is legal. The extra shenanigans doesn't change that question.
#1 by itself is not illegal. When companies move a factory overseas they also just fire everybody. Not nice, but not illegal.
It's not illegal in se, but it is illegal if it's done in order to retaliate against employees for organizing, in order to target union employees, in order to undermine a union, etc.
There's a question of enforcement, and there's the challenge of proving that to the requisite legal standard. But assuming the facts are established, it's actually a very clear violation of the NLRA.
They were told if they didn't sign a worse deal they would be fired, causing a fight with the union.