This is something you can easily find out by looking at the admissions websites of the medical schools you are interested in. Each school has its own policy on this. Basically, you will find that schools do one of a few options:
1) MD/PhD and MD committees simultaneously consider your application for their own respective programs. On your interview day, you are interviewing for both programs. Columbia comes to mind.
2a) MD/PhD considers you first, and if not offered an II or an acceptance, your application is automatically forwarded to the MD committee. Most schools do this.
2b) MD/PhD considers you first, and will notify you if you are rejected. You must then send a written request to be considered by the MD program. Cornell Tri-I does this.
3) A rejection from the MD/PhD is permanent. Johns Hopkins for example, or UMass if you're not a Mass resident.
As an anecdote, you will be at a severe disadvantage if transferred from MD/PhD to the MD programs. I was rejected from 5 MSTP programs, and was very quickly (within 3 weeks) rejected from the MD programs at the same schools. So you might want to consider applying MD only if you have doubts about your application.