> If you change your lifestyle temporarily, the effects are temporary.

Not quite.

If the lifestyle deteriorates, then weight regain is expected. However, people might regain only a fraction of the weight lost. It is possible for some of the weight loss to be permanent.

Also, even temporary changes in lifestyle can produce benefits that outlast the intervention, like lower risk of disease such as diabetes. Exercise is notable for providing numerous benefits that persist even if weight is completely unmodified.

> If you change your lifestyle permanently, the effects are permanent.

No. It improves your odds of success, but does not guarantee it.

Body adapts significantly over time when subjected to weight loss. There are compensatory physiological adaptations that favor weight regain, such as changes in energy expenditure and appetite. Even with sustained effort, maintaining the initial weight loss may become progressively more difficult because of those changes. This shows up in research as results like mean weight loss of ~9% at year 1 and ~5% at year 8.