We moved from /users/* to /u/* over 4 years ago:
https://github.com/discourse/discourse/commit/45a257815a0181059de91101fa4f9c6f8aa9b3c7
Both routes were still supported in the server-side and on the client-side router during all this time, but as we are doing heavy work cleaning our client-side code, long deprecated routes are being removed to simplify the code.
Our official commit log is https://github.com/discourse/discourse/commits/master, so if you are creating your own customizations you will want to read the diff between the commit you are running and the new one you plan to update to.
What most people do is using a staging environment with identical configuration, where you can roll updates first and run tests to see if something broke.