It is up to you. If you have an established relationship with your doctor and your specific problem can wait (your doctor's office can assist in figuring this out), you may want to go ahead and wait. But, chances are if this is a partner in the same medical group, the partner will share similar care values as your primary provider. So, in the end if you do not have an urgent need it is really up to you!
Depends. If you are comfortable with your established doc then wait. If there is something urgent that needs immediate attention you should get in quickly.
Wait. It all depends on your comfort level with the dr. If you really like and trust the dr, then wait. If it is not an emergency and you can wait, then wait. If it is an emergency and you cannot wait or if it doesn't matter, then see the younger guy. You can also always ask the nurses or receptionist at the office who they would rather see for themselves and make your decision.