How to Update an Oppty Product Rev Record

I have two customers who have merged.  I need to change the account that is associated with the OPR records for an opportunity.  I've created a report to identify the rows involved.  I figure I need the OPR External Unique ID.  What other fields do I need and what would they map to in order to upload a file and change the account?  If it matters the associated opportunity is not changing, I simply need to change the account on the OPR records.  I've tried numerous times with different mappings but keep getting the following message:

Unable to set parent