The warehouse that was right for direct-to-consumer volume is often wrong for wholesale. Retailer routing, carton labeling, and ASN timing are a different job, and a 3PL that cannot do them turns your best accounts into deductions.
Ask which of your target retailers they already ship to and get the account names. Ask who generates the ASN and how quickly. Ask what happens to a chargeback caused by their error, and get the answer in the contract rather than the sales call. A 3PL that already ships to your retailers has solved problems you have not encountered yet.
Selecting a 3PL is the easy half. Moving inventory, cutting over integrations, and keeping ship windows during the changeover is where brands get hurt. Plan the move outside your peak, run both sites in parallel where you can, and do not cut over the week a major account expects its first shipment.