Coincidentally we had the same issue today. To prevent this from happening in the future, I wiped out all the "salesperson" data in the contacts/shipto by doing an import with a blank column. When this field is blank the order will default to the salesperson on the invoicing tab. Unless you have different reps for different locations for the same customer, then leaving this field blank is much easier to maintain. Wish I could disable the field (and a few other fields).
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Susan Tennier
TDL Canada
Trenton, Ontario