Nothing to stop it, although there is less remedy as this will be after Completion and no delay damages would apply so no penalty for the additional works if they are "late".
In practice this would be by agreement - the Parties should agree the value of the works so there are no arguments. Contractor has to follow an instruction but difficult to see here what remedy the PM has if the Contractor refuses to do it without getting price agreement first.
The Defect Date will not change so the liability for defects would be shorter on any of these instructed works as the Defect Date will have already been triggered by the Completion Date.
In practice this would be by agreement - the Parties should agree the value of the works so there are no arguments. Contractor has to follow an instruction but difficult to see here what remedy the PM has if the Contractor refuses to do it without getting price agreement first.
The Defect Date will not change so the liability for defects would be shorter on any of these instructed works as the Defect Date will have already been triggered by the Completion Date.