Once a shipment has been delivered and paid for is it still an option to have the importer changed or corrected?

Yes, If your shipment has been released by CBSA but not yet acquitted then the BN# can still be changed with an RMD correction.

If your shipment is already acquitted (meaning duties/taxes paid to CBSA) then a letter to correct the importer can be submitted – this does not change the internal status of the file with CBSA themselves, the stamped letter is provided to the original importer so they can use it for their accounting back up.