I have an older Trek 720 that currently runs a 7 speed cassette. I believe the rear spacing is 135 because I put one of my newer road bike wheels into the stays and was much narrower. What I'd like to do is swap the 7 speed freehub body with a 9 speed body so that I can use the older wheelset(they are much more heavy duty). Everything is shimano and hoping I can make the swap without any issues. I'm hoping for some comments on whether or not I can do this?