It is not random. The Arizal changed whatever he did after a thorough understanding of the Nuschaos.
We keep all kinds of traditions because of She’al Avicha. It is very important to be part of a Mesora and not to go off on your own ways. However, the Arizal understood his position and he was guided.
As for us, if you are a Talmud Muvhak of someone and you want to take on Hanhagos from him, I don’t see why not. That is also not a random direction. The Baal Shem Tov spoke of the dangers of Haholech Baderech Yechidi. Although Misnagdim at the time accused him of just that, obviously he understood that he was not alone.
There were many Tzaddikim and Talmidei Chachamim that started new Minhagim. They understood that they can do that. It was from Dveikus and they were able to check that they were not going out alone.
The Teshuva most quoted in regards to changing the Nusach is the Chasam Sofer. However, if you read it you can see that he has no real Halachic complaint. He mainly deals with the fact that everyone has their Shaar and you shouldn’t change it, in order to connect to your proper channel.
Being that it is not an Halachic issue, only a Sohd issue, I’m sure it’s not so hard to accept that someone will choose to follow his Rebbe. Besides, the Chasam Sofer is addressing the idea of picking your Nusach. Now that it is well established and you want to change your Nusach to fit with your new surrounding or your Rebbe, that is not at all like picking a Nusach — which takes me back to what I started with. Where was I …