Client created a PO with many Detail lines, from Vendor #1. After we Posted the PO, we realized that the whole PO should have been entered referring to Vendor #2.
Nothing yet had been received.

I figured that the easiest way to make the change would be to go into the Order and simply change the Vendor code on the Header tab – “Vendor XXXXX used but not defined” message.
So this does not work.

Next I figured that we would use the COPY button on the PO’s main screen, and then change the Vendor Code in the Header. Same problem.
So this does not work either.

We finally got it to work by editing each Detail line and one-by-one, changing the Vendor Code.
We then changed the Vendor code on the Header, and finally Posted the Order.

Note that this is a busy office, a lot is going on and it is not unusual that mistakes may happen. So we need to be able to make edit changes when needed.
Our question: Is this the way that we should make this change to the PO?
Is there not a quicker way to update the Vendor on the PO and all the Detail lines more efficiently?

Sam:

You should have no problem changing the vendor on an open PO or during the copy of any PO. I believe there is an issue with changing the header vendor not changing the selected vendor on the detail lines depending upon Company Profile options but I don't recall that it gave the error you indicated (in fact it gives no error but doesn't do what you think it should). Different setup options might overcome this.

To start it would be helpful to know:

What version of PO?

In the current version on the Company Profile System Options tab (may be in a different location in older versions) there is an option "IC Item vendor check". Which option is selected?

Doug:
I am using PO 9.2A (150217) and I tried this with Sample Company.
The "IC Item Vendor Check" is set to Manually.
I changed the field to: Use selected vendor, but I do not see any difference.

We are not having an issue with changing the Vendor on the Header.
What we would like to do is take a posted Order that refers to Vendor01 and change the Header, and all the Detail lines as well, to Vendor02.
We can only do this currently by editing each line one at a time.