I know that there are many guides that show how to do the mods, but most of them permanently damage the old c-stick, meaning you cannot convert back if you don't like it. The process is almost identical on the smaller new 3ds.
http://www.ebay.com/sch/i.html?_odkw...thumb&_sacat=0
https://www.ifixit.com/Teardown/Nint...Teardown/36346
http://www.ebay.com/sch/i.html?_odkw...thumb&_sacat=0
https://www.ifixit.com/Teardown/Nint...Teardown/36346