Bear in mind that you may not be able to due to funding issues. Most residency programs receive payment for residents through HCFA (Health Care Funding Administration). HCFA limits the length of training that they will pay for to the length of the residency you first start out in. This is designed, I am told, to prevent people from switching residencies because if you have no funding available programs will not hire you.
However, we all know people who have switched or who came back to do a second residency. This may be on a case-by-case basis (ie, if the program is well funded, the loss of HCFA funds may not be a big deal to them if they really want you) or perhaps the rules only apply to switching between specialties (ie, once you finish a program you may be free to start another but can't change midstream unless you apply for programs with fewer training years).
At the risk of being chastized for moving threads to their appropriate forum, I'll move this over to Rotations and Residencies. 😀