According to ABPS, it is possible to enter an independent training program after as few as 3 years of general surgery, provided those 3 years are at the same institution as the independent plastic surgery training program. However, completion of a formal residency in general surgery, urology, ENT, orthopedics, neurosurgery, or OMFS (+2 yrs G-surg) at ANY institution will fulfill the prerequisite for any independent training program.
so once you've completed a residency, you can go wherever you want. But if you want to transfer to PRS in the middle of your G-surg residency, you can only go to your current institution.