Hello Ed,
Yes, you should be able to change the vendor on a PO.
You need to be sure that, if you're making the change on the header tab, that you also visit the details as changing the header doesn’t automatically repopulate the vendor associated with each individual detail (you can have multiple vendors on a PO).
What is your ‘IC item vendor check’ option set to in his Company Profile, Interface Options tab?
What’s your ‘Allow vendor change on invoice’ option (same tab) set to? Enabling this allows you to change the primary or secondary vendor for a PO. Per the help:
If you select this option, you can change the primary or secondary vendor for a PO when you receive goods. This is especially handy when goods are shipped by a different secondary vendor than you expected or if you order goods through a central agency which then distributes the order to one or more vendors.