hey @mg2017 if you're dealing with non-inventory items, a quick and dirty work around is to put in a negative quantity line on the Vendor Bill to negate the effect of the line in question. Then add a new line with what you want the item to be.
Example: you have a PO with one line. Item WIDGET1, quantity 1, rate 10 bucks. You convert to a vendor bill, but you know WIDGET1 has the wrong account or something. Add a line to the vendor bill WIDGET1, quantity -1, rate 10 bucks so you'd now have a 0 dollar vendor bill if you did nothing else. Add a new line with item WIDGET2, quantity 1, rate 10 bucks, etc
It's ugly and time consuming to some extent, but it works and doesn't break the PO link. Ok process if it's fairly infrequent. If you need to make a change like this 20 times a day, then not probably feasible. I'm not sure I'd try this with inventory items either. I have a feeling it would screw up COGS. Doesn't hurt to try it though; in a Sandbox of course!!