The user interface is a sacred thing. Users learn how to navigate it, and they build a familiarity which allows them to quickly perform tasks automatically, without thinking about the task. When you make a change to that interface, it had better be for a good reason, because you risk frustrating the user by impeding or breaking an established workflow.
Detrimental user interface changes are often seen in consumer-facing software, where the stakes are lower—in general, nobody loses their life when Google pushes out a more complex and less-performant Gmail UI. Contrast this to the healthcare industry, where a user interface change could confuse medical staff, impeding their ability to perform patient care.
Interface changes are often (but not always) a part of a software platform’s growth; platforms must evolve over time, and sometimes that involves an interface change.
If you must change the interface, take care to be kind to the user. Give the user control, where possible. Err on the side of breaking as little functionality as possible, while maintaining or improving performance.