Tip: Before you merge, review invoices for both the old and new supplier to find and resolve duplicate invoices. The merge process doesn't transfer invoices if the merge results in a duplicate invoice for the new supplier.  All invoices: In addition to transferring all invoices, this option also transfers associated payments.  Unpaid invoices: This option excludes partially paid invoices and partially applied prepayments. If you have such invoices and prepayments, you must transfer all invoices. As an alternative, you can complete the payments and apply the prepayments before merging. Note: Once the merge has completed, it can't be undone.
When you perform the Merge Suppliers task, you have the option of transferring all invoices or only unpaid invoices.
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What happens to an invoice when I merge a supplier?