If you upgrade your application to 7.4, all the code doesn't have typed properties.
Only when you then add them after the upgrade you should have tests for that or a static analyzer that tells you the changes you make are correct.
Only when you then add them after the upgrade you should have tests for that or a static analyzer that tells you the changes you make are correct.
Still, static analysis is necessary when you upgrade dependencies and for your own code. For example for seeing whether adding a type to a property breaks code somewhere else. Discovering type errors at runtime is often too late.
Luckily there are good static analyzers for PHP.