Gentoo has, since long, had a GCC Upgrading guide. A long time ago, upgrading GCC required quite a lot of side activities and was often considered a risky upgrade. But times change, and so do the GCC upgrade cycles. Improved compatibility as well as a better understood impact made GCC upgrades a lot less painful. Sadly, our documentation didn't reflect that as much - the last update on the GCC Upgrade guide was from 2008.
Today, after some clarifications from gentoo-dev, I've updated the GCC Upgrading Guide to reflect the improved situation. I'm hoping this will help clear out the uncertainty surrounding GCC upgrades. As always, comments and feedback are always welcome.