Joining the registers.

[Charles Federle]

I think it is fine that you have two playing positions each with their own comfort zone. The key is increasing the gray area between the point of no return. This makes it so you can choose your body position based on the context of the piece. If you are going up for one or two high notes and have to change how you hold the instrument this is making you do alot more work then is needed (the same is true if you are playing up high and need to grab a few low notes).

There are a few exercises that can help you expand the gray area. Something similar to shifting drills (also known as vomit drills) can help you become more comfortable playing out of your comfort zone, and expand the gray area in your holding of the instrument.