If your system changes in any of the rounds from NIT to IIT or vice versa, you need to report the corresponding nearest RC. However, after the 7th round - your final college will be decided by then - you are supposed to report at your allocated college. The duration for this depends from college to college I think. From what I remember, it was around 3-4 days in our year.
I hope this was your doubt. If there is anything else, please let us know.