How to update from bash-2 when all the ebuilds on your system use bash-3
----------------------------------------------------------------------

Download a stage1 from any mirror. It's roughly 25Mb.

mkdir stage1
cd stage1
tar jxvfp /path/to/stage1-tarball.tar.bz2
mkdir usr/portage
mount -o bind /usr/portage usr/portage
chroot . /bin/bash
quickpkg bash
exit
cp usr/portage/packages/All/name-of-bash-tarball.tbz2 /usr/portage/packages/All/
emerge -Ka bash