On the bright side, bin associations can be done via csv import, and if you set the preferred bin field (also via csv), it will autofill your fulfillments/receipts going forward. It's not a huge undertaking from a system standpoint, but it's tedious and will probably involve a lot of inventory counts. We didn't have a multi-location inventory system when we started our NetSuite implementation, and had to do a full inventory count to separate the items out between warehouses & bins. It was a pain, but so worth it in the end.