You could try getting into the Postgre SQL and doing an update statement on the system_users table? it isn't able to be shown by default and probably isn't support, so make sure you take a backup before attempting to do an update statement.

If you have the ability to set up a test cluster/machine, you can restore a backup to that and try it, or take a snapshot if its virtualized before doing the deed.

Glad to help, you've got to love the old Iowa school domain names.

I haven't fully implemented the api, but the basics aren't too bad. Take a look at the link below for how to send the API call, I'm sure if there are programmers on staff, they could help to implement this, but alteryx may also be able to this for you.

Basically you need to log in and get your authorization token, then you use that token with any further calls to authenticate.

You will then need to get your users on a site, change what  you need, then repost with update user.

REST API In 5 Minutes-No Coding - Alteryx Community

https://onlinehelp.tableau.com/v2018.2/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#Sign_In%3FTocPath%3DAPI%2520Ref…

https://onlinehelp.tableau.com/current/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#Get_Users_on_Site%3FTocPath%3DA…

https://onlinehelp.tableau.com/v2018.2/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#Update_User%3FTocPath%3DAPI%252…

TabCmd can do this pretty easily.

This video helped me a lot. It walks you through all of this.

You’ll have to install tabcmd (or do this from the tableau sever.) 

You’ll need a .csv file that has the people you need to modify. You MUST follow the csv guidelines here.  https://onlinehelp.tableau.com/current/server/en-us/csvguidelines.htm Leave the password column blank.

The createusers command actually modifies existing users.  I just checked and confirmed this.

So you’ll run CMD.exe as an administrator. 

You’ll change the directory to wherever tabcmd is.

cd C:\Program Files\[location]

You’ll log into the server.

tabcmd login -s http://tableau.site.com -u jsmith -p password

You’ll run the create users command

tabcmd createusers "c:\location\folder\users.csv"