one clarification

Yes, changes can be made in core field

Expert advice on this is recommended. In GST registration certificate, you can anyway add product HSN under Goods section. Many companies do business of Trading, Manufacturing and Jobwork services all together under same GST, but have same product category/sector. Courier and Poultry are different business sectors, not sure, may be not allowed.

Thanks sir,

Yes he can add the new business with same gst