NEC ECC: Project Managers Instruction or new contract to be formed ?

Are the parameters defined for the extent/scale/nature of work deemed to be acceptable for instructing under an existing contract and what work differs so greatly from the original contract a new contract should be formed ?

In accordance with clause 14.3 the Project Manager can give an instruction which changes the Scope (Works Information). Changes to the Scope is generally regarded to exclude the introduction of new items of work not directly associated with the original Scope eg if the original Scope was for Heating and Ventilation then an instruction to install Lifts would not be valid under cl 14.3 and anew contract would be required.