I think you're talking about scope of your certificate that not including the new packaging material,
BRCGS is asking always to add the packaging material into the scope of the certificate and include it also into the HACCP plan and supplier approval,
there is only one way as far as i know , which is contacting your certification body and telling them the case and they could get for you a solution after discussion with BRCGS if needed.
Into the same time: is the new packaging material not mentioned or used before by your factory ? if material used before but in different size or different shape , then you shall only check your scope of HACCP and if mentioned then the size will only added , if not added or used before then contacting your certification body will be the best solution.