Iphone 6s battery

i sent in my iphone 6s for a new battery. They have said they have sent me an email telling me they have sent a ‘replacement product’ and given me a new serial number.

Does this mean they have sent me a different phone back? if so will it be a refurb with new parts? or has the serial number changed because the battery has been replaced? New IMEI number as well. Both serial numbers start with an F.