One of the cardinal rules of writing a good book is carefully determining who (or what) is your best choice for a main character.
Think of the Harry Potter Series. What would it have been like, say, if Hermoine had been the lead or Ron? You would have come away with an entirely different experience, simply by a shift in point of view, even if NOTHING else about that story were changed.
If you shift POVs in your story, you must ask yourself who is the best “eye” for a given scene. It should obviously ALWAYS be the one who gives the most to a particular scene. And you discover that by asking yourself the simple question: what am I trying to accomplish here?
One final word of wisdom: never make a side character more interesting than your lead, regardless of the POV. Your readers will wonder why you are making them tag after the less interesting character in the room.by