The old angry route does not get deleted. What you are building is a rival route beside it, and at the start the new one is small.

What the research says

A meta-analysis found that mindfulness-based cognitive therapy reduced relapse in people with recurrent depression, and the programme is built around people continuing the practice themselves. The evidence is for relapse prevention in depression rather than anger, and it says nothing directly about brain tissue. What carries across is the design: the thing you keep doing is what holds the gain.

Strength of evidence: one line of research. A single study, or a theoretical account.