Updating Rails is one of the most painful processes ever. There is no real way to be sure it works. You just have to have 50,000 unit tests, cypress tests, and then hope and pray.
On any sufficiently large and old app, something will always break in a way that isn't caught by your tests or manual testing, and only shows up after it goes out live.
On any sufficiently large and old app, something will always break in a way that isn't caught by your tests or manual testing, and only shows up after it goes out live.