Try PO.ChangeVendor â€“ Post processing.
You might want to store the comments in a User Defined Field, in case Purchasing needs to use the PO Header Comments for other things.
From: email@example.com [mailto:firstname.lastname@example.org]
Sent: Monday, September 21, 2015 11:53 AM
Subject: [Vantage] BPM to pull in comments to Order Header
HI. Epicor 9.05.603 here. I am trying to write a BPM to pull in the comments from the vendor comments to the PO header. I tried to set the poheader.commenttext to vendor.comment but they don't seem to be connected in the PO update method. Am I looking at the wrong place? I don't know the programming language so I am working with the pre-made options.