Interesting. They just migrated from Tableau Server 7 to 8 and brought it up within the last day. I can access the server 8 via URL and view workbooks, permission, data sources are refreshing, and so on. It does appear to be configured and functioning as expected.
The "Server" name field is protected in the workbook.
I opened up a different workbook and the "Server" name field is not protected.
So, what is different between the two workbooks? I assume figuring that out may lead to an understanding of why you cannot change it in the one workbook and can in another. Can you post a packaged workbook file for the one that isn't working?
Here's the solution. The problem as I described it above is accurate. The question "what is different between the two workbooks" got me thinking. I opened up the *.twb file in a text editor. I searched on the sting online.tableausoftware.
Two occurrences. The first occurrence was xml:base='https://online.tableausoftware.com' in the <workbook> at the top of the file. The second occurrence was in a <datasource> tag. This gave me another clue.
I went into "Edit Connection" on the data source I was looking at in the XML. I found that when I am using the workbook locally, it is wanting to use a data source on my online site. This is the key.
To fix it, I added a new data connection, using an identical .tde to what was on online, except local.I then replaced the data source referencing online with my local .tde and closed the data connection to the online .tde. Problem solved.
This image shows what the connection looked like prior to my closing it:
I think this scenario is non-intuitive enough to merit inclusion in a knowledge base article.
A takeaway is that exploring a .twb in a text editor can be helpful in identifying the source of unwanted behavior in the GUI.