Clause 36 is really aimed at bringing forward Completion, as in the whole of the works.
Is there a requirement in the WI to complete the relevant element of works by a specific date? If so, you could change that as a PMI under 14.3 which is then a CE under 60.1(1). If there isn’t anything in the WI, you might consider introducing an additional constraint, which would be under the same clauses, but you’d need to consider what the implications might be, before doing so. An instruction for a proposed change (61.2) might be worth considering, in order to avoid issuing an instruction which costs an absolute fortune.
As is so often the case, the full answer depends upon the details of the situation.