Vote No
The change was unilateral in that NATS made the change without agreement from the Union. The change had to be made because legal advice was that those benefits became illegal once the age discrimination legislation came into effect.
I've no doubt that NATS management must have thought Christmas had come several times at once because it allowed them to make changes to pension benefits and the redundancy scheme that they would never ever have been able to make previously but it would be wrong to suggest that this is evidence that NATS was targetting the pension scheme and renegading on previous promises.