1The 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.
Expect the old path to show up when you are tired, ill or caught off guard, because it is still there and it is well worn. That is not proof the work has failed. It is the ordinary shape of learning something over the top of something older, which is also why the practice carries on after you feel better.
Islamic evidence
God does not let the rewards of those who do good go to waste (Quran 11:115). Worth having on the day the old temper returns and it feels as though nothing you did counted.
“Be steadfast: God does not let the rewards of those who do good go to waste”
Qur'an 11:115
Psychological evidence
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.
Piet J, Hougaard E. (2011). Clinical psychology review · doi
Review detail
Why it works. New learning sits alongside the old habit rather than wiping it, so it stays strong only by being used.