Ability to update supplier on Drop Ship sales order or purchase requisition/order

Summary:

We have a business scenario where at time of sales order creation, user don't know from which supplier they are going to get the service drop shipped but they know that it is going to be outsourced. So we want a drop ship purchase requisition to get created and once supplier is finalized after offline negotiations, either buyer should be able to change supplier while creating PO from PR or a sales person should be able to update supplier on sales order lines. Is there any way that we can achieve this requirement?

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