Well, given that I saw people doing this a couple of times already I probably will add something like that.
The way to prevent opkg upgrade is just avoid running it To be honest I don't really understand why would you want to run it in the first place - in no place in the docs or community forums there's a recommendation of doing anything like that
The upgrade process for specific packages (there's no process for the whole system) is just "opkg install packagename", sometimes with "--force-overwrite" if opkg wouldn't want to do that. For the kernel the process is a bit more complicated and I've described it in the blog post: http://alextgalileo.altervista.org/blog/install-kernel-from-repo-onto-edison-official-image/