Change in gst no from proprietor to partnersip

dear sir,

we have acquired a firm last year in 2018. Before that firm was running under proprietorship we have changed it to partnership firm and get new GST number for same. Then we have closed old GST number. So, now queries raised from one old customer to file gstr for 2017-18 in B2B . At that time we were not operating and old GST number is closed. So, what to do now?? customer is asking for credit. Is there any way that we can give any proof of gst return or can amend B2CS to B2B??