If you are a Subcontractor to the Contractor then presumably you would be under an NEC ECS form. A change in ‘scope’, Subcontract Works Information (NEC3), Subcontract Scope (NEC4), would need to be formally instructed by the Contractor to have effect under the contract. This ‘change’ would be a compensation event, except where you have design responsibility and you have requested this change.
If you have not requested this change then the matter should be instructed by the Contractor under sub-clause 14.3. As is often the case, however, the change may have been incorporated as part of a revised drawing issue from the Contractor’s designer. In this instance you need to understand if the person issuing the drawings has authority under the subcontract to change the scope of works. If they have such authority then they should have notified the matter as a compensation event at the time. If they didn’t notify then you would notify the matter as a compensation event under sub-clause 61.3.
If you are not sure about the situation I would advise giving an early warning by notifying of a matter which could ‘increase the total of the Prices’, which is effectively saying that it would be a compensation event. This is under sub-clause 16.1 (NEC3) or 15.1 (NEC4). This should lead to a risk reduction (NEC3) or early warning (NEC4) meeting at which you should be able to clarify the ‘change’ and decide upon what actions are appropriate.