I don't see why the 10mm to 40 conversion barrel wouldn't work, but I am not an expert or gunsmith. I very well intend to get a conversion barrel for my 21. I would rather have it and not need than the other way around. 40 S&W is easier to find than 10mm for sure. And, in most cases, cheaper. I picked up a 100 round value box of Rem 40 at walmart when everything else was sold out.
US Army Veteran, The 21 Club #7663, The 10 Ring #7663.
Amateur Glock Modification Expert