It looks like the contract has gone into a 'painshare' situation, for whatever reason, and the Employer is unhappy with the Contractor's performance. In my experience the fact that a Contract is in a 'painshare' situation doesn't necessarily mean that the Contractor is not performing. There could be numerous reasons for this, not least a PM who makes their own unrealistic assessment of CE quotations.
There is nothing to prevent an instruction to change the Works Information under clause 14.3, which removes scope and then this becomes a CE under 60.1 (1). The Defined Cst is assessed under 63.1 but you cannot have a negative time impact on the Completion Date only the planned Completion, thereby increasing terminal float.
If the Employer instructed the PM to remove 'significant' scope and subsequently awarded this to someone else, then the Contractor could have cause to consider action outside of the contract. The Employer, however, could terminate if they so wished for 'any reason' under NEC3 (although this has changed under NEC4), so they could pursue this course of action and follow the applicable procedure under clause 93 to assess the amount due to the Contractor.
Be aware that if the above course of action does go ahead the Contractor would likely pursue matters further, including the possibility of dispute resolution. Informal discussions 'off the record' between the Employer and Contractor at any stage of a project are often useful to resolve or mitigate the effects of potential conflict situations, especially if the Employer intends to terminate or request that the PM instruct to remove scope.