NEC3 61.1 has the sentence - ‘The Contractor puts the instruction or changed decision into effect’
NEC4 removes this statement.
I would read the statement as being essentially ‘get on with the works instructed alongside producing the quote and going through the implementation procedure’
If the NEC4 no longer has this statement, does the contractor have to start working on the instruction or are they able to hold off until once the CE is implemented?
In summary, no they are not able to hold off. Both NEC4 27.3 and NEC3 27.3 requires the Contractor to obey an instruction (which is in accordance with the contract). NEC4 63.9 has similar wording to NEC3 63.7, in that:
“The assessment of the effect of a compensation event is based upon the assumptions that the Contractor reacts competently and promptly to the event and that any Defined Cost and time due to the event are reasonably incurred.”
“Holding off” could result in the assessment being reduced due to not reacting competently and promptly.