The most important part of your choice is to be sure you select a genuine Registered Patent Attorney. The U.S. Patent Office gives a special bar exam to qualify and register its attorneys to appear on behalf of the client. There are fakes who might be foreign attorneys or are what we call "invention promoters." The Patent Office tries to find the fakes, but the fakes often hide behind the inventor so that it looks like the inventor is entirely representing himself. This is not fair to you and puts your case in jeopardy.
Any time you wish to apply for a patent, it is advisable to be represented by a Patent Lawyer. The Patent Office operates by numerous complex rules and procedures. A Registered Patent Attorney is your best resource to get through the patenting process.
Yes, you can represent yourself in your own case. However, experience shows that even a beginning Patent Lawyer might need 18 months of supervision to gain the needed experience to do a good job for clients. A "pro se" inventor (representing himself/herself) has a long, uphill struggle to acquire similar ability to do a good job.
Yes, the change of Patent Attorney is done by filing a form called a Power of Attorney in favor of your new representative. Your new Patent Attorney will handle this.
Ideally, you are the technology expert about your own invention. Start by giving a detailed, written description of what you have made. Drawings are desirable, and even rough sketches are valuable. Explain how and why you made the invention, highlighting problems in your technology that your invention overcomes.