Can I change class?
Your class is set once you pick one on a new import/creation for ME3. Unfortunately, your class is retained when you start a new game+ so you cannot change it. Bonus powers, however, are retained by profile so you can pick a different bonus power.
You can change your class when importing a Mass Effect 2 save, before the tutorial.
You keep your level, but (understandably) you will start out with all your skill points unassigned.