They have a retrofit model cailing fan box, it actually works nicely. The biggest problem I've had doing this type of work is with the Bx's being too short to make up the new box.I've found the best way to do this is with a hacksaw blade holder, which will fit into the opening of the ceiling, strip the bx long enough to fit into the box. apply heat shrink tubing over each conductor is what I personally like to do.99% of the time the fan housing will cover any plaster which happens to break free during the work.
