OMG. OP better have a good reason for switching... No, wait... OP better have a great-knock-your-socks-off reason for switching. Not just, "I want more pay or I want more responsibility." Switching from nursing to MD or DO is not unheard of, but many adcoms will scrutinize the reasoning behind the switch. Why? Because you are taking away training that could have been used for someone who truly wants that position as there are a shortage of nurses, PAs, etc. already. But switching from a PA to doc, many would frown on this as there are even less openings for PA students in this country than for nursing students. The only thing that I can think of that may warrant some change is that you REALLY feel passionate about surgery or something that a PA doesn't do. If you want to go into EM, IM, or peds, you have a more difficult challenge in presenting your case.
If you can't answer hopeful's bolded question and get reasonable responses here, you may have a pitiful chance at convincing an adcom. Why didn't you apply for med school before? Why PA instead? Do your best to answer these questions here and we may be able to spot problems in your app before you shoot yourself in the foot in the real deal.