customer / supplier merge

Hello team,
after upgrading to R12 we faced the situation of having duplicates.
both  customers and suppliers are in hz_parties table; no update is allowed on (eg) a supplier if same VAT code is present in one customer with different party_id.
so we tried supplier/customer merge.
We have cases of suppliers (which are our partners, actually) that should be merged with related customer (same VAT)
Having set the supplier as the winning party we tried the merge process.
in error log we faced veto rule
[..]Please pay all unpaid invoices and close all open purchase orders, and submit the Supplier Merge Process[..]