Have A Go had it right. Using Andersen boxes works. I have replaced Pella boxes in 4 windows with Andersen boxes so far. As HAG said, just measure the window to pick the correct box (the first number in the Andersen box nomenclature means pounds, ie. 637 is a 6# box with a 37" cord). Take the top molding off the window to expose the old boxes, pry them out (will probably come out in pieces). Since the Andersen's are smaller you need to position them so the cords are lined up correctly. You will also need to drill holes to hold the new boxes in place (there is a rather thick metal plate to drill through in my windows). I put two screws at the end of the Andersen boxes to secure the new box (used sheet metal octagonal head screws) and drilled one hole through the cord-end of the box (there are already two holes in the top cover so you can be sure to miss the cables and internal bits). I drilled through the front hole because it was easier to do. Once in place, to attach the cords I left the Pella swivels in place but just drilled holes in the side of each sash to insert the Andersen cylindrical cord end (just as HAG said, I used a 5/16 bit which was perfect). Worked great. I have 4 done (did 2 on Sunday) and about 6 more to go. Wish me luck. I really want to thank Have A Go. He may be a "Newbie" but he really got my DIY spirit back in gear. Thanks !