Hi,
This was a common situation at ArcGIS 10.7.1 and before. We'd very often see just 'arcgis' as the Web Adaptor context and then it reference the Enterprise Portal and hosting server. At 10.8.1 there was a change which meant that a separate Web Adaptor was needed for each software component:
https://enterprise.arcgis.com/en/web-adaptor/10.8/install/iis/getting-started-with-the-arcgis-web-ad....
In an ideal world, your deployment consultant shouldn't have configured that way at 10.8.1. It was the first release like that, so may have been a mistake. There are fundamental changes at 11.1 with the Web Adaptor. It's been upgraded to .net 6 and there have been some reliability issues. As a result of all of that, I would not recommend upgrading "as is" with a single Web Adaptor.
Changing the hosting server URLs also used to be an unsupported activity, however, I believe that this is something that can be done with the help of Esri Professional Services.
Another option would be to not use a Web Adaptor and use a forward proxy instead. That would allow you to upgrade without changing the context of the server/portal, which will preserve your content. I see other have responded in the time I've been preparing this, and their comments are completely valid. It depends one what else is in your 'Enterprise' that can be used to suppor that option.
Scott Tansley
https://www.linkedin.com/in/scotttansley/