Is the upper part of the rear bar on a BF all the same between XR and base models?
In otherwords can I put my lower section (XR style) onto the upper section of a base model BF?

hi mate yes it is. one problem to watch out for though. most ba and bf have a 2 piece bumper, where the lower piece of the skirt can be changed easily between an xt, sr, and xr. however some have a full one piece bumper. might want to check this out before purchasing an xr lower half. i havent been able to find a decent reason why some got one piece and some got 2 piece bumpers either, it seems to be hit and miss. i wanted to do the same thing to my ba series 2 xt. i bought the lower piece and had it painted to find i had a one piece bumper and needed the top half too.