Note do not let this scare you, I have not seen many failed updates with linux, and I updated cleanly with no problems.Tim,
What cpg means is if you update your OS to F10, the Amahi software repository would automatically change from the F9 location to the F10. This is where the Amahi Home Server software is pulled from during application and platform installs/updates. This is typically transparent to the user and that's why it's automatic when you upgrade your OS.
If you want to upgrade your OS from F9 to F10, then you would need to download the DVD image as you stated and recommend you perform the upgrade option. That is what worked best for me. Keep in mind, there are risks with doing an OS upgrade and you should back up your data. Should the upgrade fail, you could lose it all.
I hope this answers your questions.
Just make sure you iso or torrent download is from http://www.fedoraproject.org or a certified mirror from the site and you should be good.
Backing up is always essential in any kind of drastic change, so I would still recommend it.