Translations:Handbook:Parts/Portage/Branches/13/en
From Gentoo Wiki
When changing from the stable to the testing branch and performing a @world update, it is normal for many - sometimes hundreds - of packages to be updated to the latest available versions. Updates generally correspond to the upstream project's current release. Keep in mind that, after moving from stable to testing, it may prove a challenge to revert back to the stable branch. This is to be expected.