I'm a current Harvard resident and I rotated through the system. The registrar's office is actively terrible and will make your life difficult without the slightest bit of remorse. I found out about 1 month before the rotation started so just have patience. In that month usually they can accommodate most. It is the same with all other specialties.
Don't bother calling, they will yell at you. I promise, though, that the hospital staff are the polar opposite. It's a pleasure to work with people in the partners system, everyone is more than helpful! PM me with any other ?'s.