Is there a way to edit the header row of a vendor bill without the line item pricing changing?

We're trying to do some mass edits on vendor bills throughout the year where we need to change the vendor but keep the line item pricing the same. I've tested a few uploads and every way I try it it resets each item to the default pricing. These are non inventory items (they're medicare commissions) and they vary depending on where they're coming from. This means that we can't set vendor pricing on the vendor card because the same item from two separate carriers can pay differently to the same vendor, however they're listed in NS as the same item. Prices are set in our initial vendor bill upload.

I really need to figure out a way to change the vendor without changing the price, as some of these bills are for thousands of dollars with hundreds of line items. Those line items resetting can drastically change the total of the bill.

I do not have admin privileges in our live system, but I do in sandbox, so I can test to see if something works.