It seems that updating outdated systems is a quite common headache, and that often people advise to just reinstall. Recently I was updating a system that was first installed in 2005 (amd64 stable), and last updated in April 2010 (about 8 months before this blog post). Fortunately I didn't hit major problems (like "masked by EAPI" errors when updating portage), but I hit a few minor ones, and I'd like to share the solutions and the overview of such an update with the community.
Note: I update most of my systems much more frequently (monthly or weekly depending on the system). This post is meant to show that while it's not trivial to update rarely, it's not as complicated as one might think.
Note 2: I'm not sure about distributions that have 6-months release cycle, but it's quite possible they have their own problems on updates (I guess most common problems are that X, sound, or wireless network card stop working).