Simply ask your new attorney to do the work for you. The new lawyer should even do the dirty work -- contacting your old lawyer and asking that your files be transferred. If you are involved in litigation, you must also sign a substitution of attorney form, and your new attorney should file that with the appropriate court.

As with other partnerships that run amok, it is never necessary to give a reason to end a relationship with a lawyer. You hired him, you are free to fire him.