Do you have unit tests for critical functionality or complex conditional logic? Might want to start from there. It might start out really hard depending on your architecture, but once you've set it up, you should be able to refactor fast. Think of it as setting up a safety net before doing a paint or window job on a really tall building. You'll move faster if you aren't worried about shit happening.
Yes, luckily the core of the app is pretty well covered with tests and they have been a huge help in making sure refactoring doesn't break things (which it has, repeatedly)
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