When SERCO took over at an airfield in the south east of England, I was told they sacked ALL the controllers on the friday, then offered new contracts to about 50% of them the following monday. I was told this by one of the controllers who stayed on. This was in the days before TUPE of course, when employers weren't required to offer equal T & Cs.
What NSL need to do is replace all the controllers who wish to remain with NATS with controllers who are about a year off retirement.