I tried to follow the instructions at Unregistering ArcGIS Web Adaptor with Portal for ArcGIS—ArcGIS Web Adaptor (IIS) Installation Guide ... . After clicking the Unregister button, I am redirected back to the same page and the web adaptor seems to still be registered. Is it safe to pretend that the Web Adaptor got unregistered and go to the Web Adaptor on a new web server and register it with Portal?
Part of the challenge you may run into is that if the WA creates the "public" or "frontend" URL for you, (the URL clients use to reach the Portal and the URL), then it's stored in all items and within the federated server. If your new server has a different FQDN and you move the WA onto it so you can register with Portal, you'll need to handle the new URLs in all data. If it's a new machine but has the same CNAME, then you just need to make sure you use the same URL to register the WA and you'll be fine.
It's the same CNAME. You posted in https://community.esri.com/message/940443-re-rename-federated-ags-computer?commentID=940443&et=watch... with the ultimate solution to my trouble so I'll follow the instructions there.
I'm glad that in the future, if my WA machine dies, I don't need to worry.