Jo,
“Halachicly one should not change from his father’s derech, minhagim, etc. That a requirement. ”
Can you please explain then how Chassidus or Chabad came about? At some point their ancestors did not daven Nusach Sephard/Ari etc.
אלא מאי
It is far from as simple than the black and white you have stated. There are minhagim that can not be switched and there are some that can, with certain conditions. However, the main thing is you need to follow “a” mesorah. If you decide to join Chabad you need to keep all chabad minhagim. Same applies the other way around..