Update supplier site payment code

I need to update the supplier site payment method from the backup. I found a script on-line that uses iby_disbursement_setup_pub.update_external_payee to do the update. However, if the supplier default payment method code is different from the supplier site payment method code, both are getting updated and I only want to update the supplier site. For example, if both the supplier and one supplier site are set to EFT and I execute the script  to set the supplier site to CHECK, both are updated to CHECK. How can I use iby_disbursement_setup_pub.update_external_payee to update ONLY the supplier site payment method?  Here is