Interesting question. The first question is to ask, what is the advantage of attending a medical school with a great reputation? I look at it that there are two reasons for wanting a medical school with a great reputation. The first, and more important, is because you will likely get a better educational experience. I think the basic science courses are less of an issue, as the material you need to learn is pretty standard everywhere. The bigger issue is the clinical experience. At a top tier school, you are likely to work with better residents, attendings, and other medical students. This is not universally true, but should be true more often than not. At many lesser known schools, you can find outstanding residents and attendings as well - and I'd encourage med students at any school to seek out these individuals. Working with great doctors is a priceless experience - there is no substitute, and you won't really have the same opportunity after residency / fellowship. The second reason to want to attend a school with a great reputation is because it may help you get into a better residency program. This is more important if you want to go into a very competitive residency field like orthopedic surgery, or if you want to go to a top-tier residency program within a less competitive field (i.e., going to Mass General for internal medicine). The question continues to repeat itself - quality of undergrad school may have some impact (but not much) on the quality of medical school you can get into. The quality of med school may impact the quality of residency program you can get into. The quality of residency program may impact the quality of the fellowship. The quality of the fellowship may impact what type of position you get when you finish training. In the end, I think the most important factor in all of this is the quality of the individual - someone who is a good undergrad student will get into a good medical school will get into a good residency will be a good doctor, etc etc.

I think this question is similar to questions people have asked about attending D.O. schools and how it impacts career choices. Another common question is about med schools outside of the US that partner with US hospitals for clinical rotations. In general, D.O. programs are reasonably competitive for getting people into decent residency programs, but I think are at a disadvantage to M.D. programs overall. There is a much bigger disadvantage to people who attend medical schools outside of the US.

Now - assuming you are still thinking that there is a reason you should try to transfer medical schools - I will say it is possible, but I wouldn't go into medical school with a plan of transferring at some point. The few people I know who transferred in were very good canditates from very good schools and were transferring because of life circumstances - such as a spouse moving that they had to follow. It's somewhat disruptive to transfer schools because you don't know the other students, you won't be familiar with the school, etc. However, if you transfer between 2nd and 3rd year, it's probably not that big of a hurdle. As to the logistics of transferring, you'd really have to check with the particular schools. It likely would depend on how many openings the school has that year as well, so I don't know that you can count on having the option to transfer in somewhere you'd want to go.

The bottom line is, I don't think you should expect that your performance at one medical school can be great enough that you will become attractive to a higher tier medical school that you weren't able to get into out of college.