I used a hole saw to make the new hole after cutting down my sweep.Don't try to use twist bits to bore the hole, as it will splinter to much. I think you would get the cleanest hole with a annular cutter, but I did not have one of the right diameter, so I don't know for certain.
Unless it's a really narrow Avon, you will be happier with 10 ft oars. I went back and forth on the decision with a mind to all the rocky rivers I like to run and in the end went with 10'. Once you get a feel for them in lower/rockier situations there is only benefit to the extra power.
So, before you start cutting the oars, a few questions. Was this a used set-up or new? If used, was the previous owner using the 10 ft. oars successfully? How wide is the raft & frame? If it's new, why did they sell oars that long with the package if they're way too long? Have you set the raft & frame to try them? I'd make sure that they won't work before I started cutting or trading them. I say this because I have suffered with "short oar syndrome" in the past. Just wondering?????