Swap 360 boxes to streaming

I renewed my contract a couple of months ago and was hoping to change my main 360 box and second mini box to streaming boxes, but was told I couldn’t if I wanted to get the best deal. Is it still not possible to swap them without changing contact? I want to move my main box and the second box isn’t cabled in, but I’m in a rented flat so can’t drill more holes in the wall.