I\'m really confused. I want to do something that a) seems like it should be pretty simple, and b) other people must do all the time, but I cant find out the best w
I haven't tried this with an multiple repos myself, but I don't see why you can't use the suggestions from of your 2nd link.
svn merge ORIGINAL@REV UPGRADE@REV LOCAL_PATH
This effectively tells SVN to take all the changes between the original checkout and the version you want, and to apply them to your local copy.
Protip: I always use explicit revisions and include the merge command with the commit message, so that I can easily review the history and understand how to reproduce or undo changes.