Well, I'm only an applicant myself, so take this with a block of salt. However, if you switch post-bacc programs in midstream, you will have to come up with a very compelling reason as to why you made that switch. "It was too hard" won't really cut it, I would expect. They would likely be concerned that you would switch again when medical school got to be too hard.
The best solution, if feasible, is to address why you're having trouble getting the grades you want. Speak with profs and advisors and ask what they suggest. Go in for office hours, send e-mails, ask questions, get a tutor if it helps..