My understanding is that once PGY-1 is over and step III passed, it is over for good.
If you wanted to do ortho or plastics after general surgery, I thus think that you wouldn't have to start over from scratch.
I'm not much of a surgery guy, though, so feel free to correct me if you have more reliable 411.
Switching specialties, while not common, does happen. It can happen part-way through your first residency, or as noted above, after you've been in your field working and decide to go back. There are no rules regarding this with the exception of the possibility of your second program not getting Medicare funding for your position.
In the case above, it is very possible to do plastics after general surgery - as a matter of fact, that it the usual route. Integrated programs which match right out of medical school do exist, but the traditional and more common route requires general surgery training.
The same is not true of Ortho. As noted in previous posts, it is not a general surgery subspecialty. You may get credit for some of your general surgery rotations but Ortho requires their PGY-1s to do rotations in other fields such as Rheum, Plastics, etc. which a general surgery residency wouldn't necessarily have. Therefore in most cases you would repeat at least part of PGY-1 before starting dedicated Ortho training.
To summarize:
ENT, Ophtho, Uro, Ortho, Neuro are not subspecialties of general surgery. Therefore, switching into these fields essentially requires starting over (with the possibility of getting a few months credit for some of your gen surg training). Each of these may have subspecialty fellowships which require more training - ie, Hand or Spine for Ortho, Spine for neuro, etc.
Independent Plastics, Vascular, Trauma, Transplant, Breast, Endocrine, CT, Minimally Invasive, etc...these are general surgery subspecialties and require a residency in general surgery before embarking upon them. You are not switching fields in these but rather pursuing a subspecialty fellowship.
Being Board Certified has nothing to do with the length of training required if you decide to pursue another line of training. Therefore, just the fact of being a BC general surgeon does not excuse you from completing a full Orthopaedic residency should you choose to do so; you might get a few months knocked off but for all intents and purposes, the second residency is no shorter.
It would appear that the OP is asking if one were to be unsuccessful in getting into Ortho (or some other competitive specialty), would it be useful to train in another surgical specialty. The answer is yes and no.
Yes in that by training in another surgical field (say doing Prelim Gen Surg), you are keeping your clinical skills up, making contacts and perhaps may even do a few rotations in your field of choice. But the answer is also "no" in that you could spend years trying to match into the competitive specialty but won't necessarily get credit for all that time. By way of example, I knew someone who desparately wanted to be a Urologist. He spent 3 years doing general surgery before he finally found a Urology position - a PGY-1 position. I think he got a couple of months credit for general surgery, but in essence, it was starting over from square one.
Bottom line is that you shouldn't choose a back-up field, Plan B, without the realization that you may have to start all over again in your field of choice.
