Changing attorneys may be a simple procedure or a complicated one depending on the specific situation. First you must retain the new attorney, who will notify your current attorney of the change of representation and request the file. If the matter is solely before the USCIS or the NVC, the new attorney must file a new G-28 "Notice of Appearance" form to be recognized as the attorney. If the matter is before an Immigration Judge it becomes trickier, because the Judge must permit the change.
If you retain an expereinced immigration attorney, he/she should know the rules.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. A consultation with an experienced attorney is always the best way to go.
As my colleague points out, your new (competent) attorney will take care of all the paperwork.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Legal disclaimer: The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.