On Dec 20 2005 I converted my lens from an EFS mount to an EF mount. Thanks to Rich Cutler who explains this procedure nicely here http://forums.dpreview.com/forums/read.asp?forum=1029&message=15346329. I tried it on the 17-85IS and it went very smoothly. It took me about 15 minutes to do, although I think I could do it now in about 3 minutes. It is quite easy, but if you choose to do it to your EFS lens, you do it AT YOUR OWN RISK.