I have a project where I am moving Thousands of documents from over 4,000 Document Libraries to 15 Document Libraries.
That project is working, where I am capturing metadata from the "old" libraries, and storing it in columns in the new Libraries.
In the screen shot below "Pre" equals "Previous" value from old library:

What I now need to do is update two Lookup columns in the library with that information:

It would be GREAT, if I could just "Stamp" the Value from "PreClientName" into the ClientName column, but that would be too easy 😐
I did try using a "Get Items" step, and filter on the Client ID, which does get the Item and the data( Using a different Document here in a test library):


But I'm getting lost on how to update the data in the Lookup column(s).
Using Update file properties to set the "ClientName ID" field to ClientName from the Get Items step doesn't work..

I get an error ( where XXXXXX is the Correct Client Name):
"code": 502,
"message": "BadGateway",
"message": "Error converting value \"XXXXXX\" to type 'System.Int64'. Path 'Id', line 2, position 26.\r\nclientRequestId: 766c89d1-cfcd-4393-bb8b-ec0bd107905b\r\nserviceRequestId: 766c89d1-cfcd-4393-bb8b-ec0bd107905b"