[tomxor]

Great advice. One issue with testing: even once you have a copy of the production DB, it's not actually a live production DB in that it's not being modified and used continuously in the same way. This can hide issues with your change.

My point is, testing on a copy is necessary, but you must combine it with thinking as carefully and thoroughly as you can about the potential side-effects, which in turn depends on your understanding of the application architecture. Some of those side-effects you can test for, others are not so obvious unless you can better simulate the production environment with noisy users who will complain when things go wrong. One strategy is to find people internally who are good at being real life simulators of noisy users who bump into edge cases.