There have been a lot of topics on becoming a psyker, but I don't see any clear rules on what it means if you lose the trait, which is certainly possible in BC. I can see a few options, not sure which are more likely.

1. You keep your psy rating and powers but you're stuck at what is almost certainly a lower power level than you'd want.
2. You keep your psy rating and powers, but you can't use them. Suddenly a life in service to Tzeentch has a lot more to offer since you'll get all of that back as soon as you earn his mark and the psyker trait with it.
3. Your psy rating and powers go poof. Any xp you spent on them is lost forever and if you regain the psyker trait you start from scratch.

I include 1 because I note the trait is explicitly stated as needed to buy the other things but not that it's needed to use them. I figure 2 is probably more likely but is there anything official?

BC corebook p.78 upper right corner, Khorne & Psykers. Keep Psy Rating and Powers but can't access them. Not the only way to play it but possibly closet to official.

Yup, Khorne and psykers box. Lose access until you're not aligned anymore.

While there are words from the book on if you lose the trait, I am confused or wondering about other approaches to this.

Surely how you apply the loss of psyker status depends how you lose the powers. Being Psyker (correct me if I'm wrong) is just a genetic mutation so could it not be feasible that gene therapy or some procedure could affect or counter this mutation?

You stop being a psyker, something fundamental has changed or someone much stronger than you is stopping you. I'd say that you definitely lose all benefits and capabilities of your Psy Rating and all Powers.

Exactly what happens and the effect of that would depend greatly upon how it happened. Most likely, I'd say that any psyker not prepared for the effect would suffer enormously. It would be like losing one of your senses.

In the case of becoming a khornite, this might be something you want, and at most, I'd say that you maybe, maybe will still be able to do basic psyniscience.