Passing tests did not prove unchanged behaviour.
The existing suite ran 647 tests with no failures or errors before and after the completed change, with two tests skipped by the project. During the upgrade it caught a stats-page crash, which was fixed. Runtime checks found a separate time-handling change.
A time at 12:00, offset +05:00, converted inside the app
Rails 7.2: 07:00 +0000
Rails 8.0, before the fix: 12:00 +0500
Both represent the same instant, but the application's output changed. We restored the previous behaviour and checked the result.