Today I got "pinged" on bug #463240 about the difficulty of upgrading a Gentoo Linux deployment after a long time of inactivity on the system. We already have an Upgrading Gentoo article on the Gentoo wiki that describes in great detail how upgrades can be accomplished. But one of the methods I personally suggest most is to do small, incremental upgrades.
Say you have a system from early 2009. Not too long ago, but also not
that recent anymore. If you would upgrade that system using the regular
approach, your system would probably be using a non-existing profile
/etc/make.profile symlink would point to a non-existing
location), and if you switch the profile to an existing one, you might
have to deal with problems like the profile requiring certain features
(or EAPI version) that the software currently available on your system
This problem is mentioned in the upgrade guide through the following:
Make sure your Portage is updated before performing any profile changes.
However, it does not tell how to update Portage. In my opinion the best way forward is to install an older Portage tree snapshot (somewhat more recent than your own deployment) and upgrade at least portage, perhaps also the packages belonging to the system set. So for a system that has not been updated since January 2009, you might want to try the portage tree snapshot of July 2009, then January 2010, then July 2010, etc. until you have a recent deployment again.
All that is left for you to do is to find such a snapshot (as the Portage tree snapshots from the mirrors are cleaned out after a few months). I try to keep a set of Portage tree snapshots available with a 2-month period which should be sufficient for most users to gradually upgrade their systems.
Considering I've used this method a few times already I'm going to add them to the upgrading instructions as well.