Correct. You could lower the amount of time it takes to do the switch by having two SD cards. One card for each server. In that manner, you would just power the device down and change the card. Each card would log into the appropriate server (no re-flash required).
Since the application is not a CMS (like WordPress, Joomla, etc…) you will need to fork the application.
The FarmBot Web App is a Typescript / React app, so you would modify the code to show whatever code you need. All of the view files are stored in *.tsx files. If you edit the file and refresh the page, the view will change.
Since the application was not meant to be used this way (it is not a CMS), you may hit challenges when you need to update the server version. You might be better served by creating your own application that reads the API or database for the server but which does not change the app itself. That way, your custom application can update independently from the Web App.
Alternatively, it is possible to download all images from your account and view them in a different application using the HTTP API.
Examples: