Title: Downgrade glibc in gentoo
Subject: How to downgrade glibc (unsupported by gentoo).

I inherited a gentoo box form a developer who lived on the edge ( ACCEPT_KEYWORDS="~amd64")

I personally find managing a stable gentoo system challenging enough.

I want to downgrade glibc from 2.16 to 2.15-r3 (stable at the time of this writing).

Since glibc is the source of everything, I found my self in a pickle:
 Once glibc is downgraded, gcc will stop working, 
 gcc not working, I can't rebuild anything,
 can't rebuild means gentoo is broken.

A solution:
REF: http://www.gentoo.org/doc/en/gentoo-upgrading.xml#old_system

Gentoo offers a path though this maze:
1. downlaod stage3
2. unpack into a place where I will chroot.
3. make mount point in chroot for /
4. chroot
5. emerge everything with target of my mounted /
6. revdeprebuild and emerge the stragglers

Go:
 mkdir /mnt/build
 tar -xf -C /mnt/build /path/to/stage3-somearch-somedate.tar.bz2
 mount --rbind /dev /mnt/build/dev
 mount --rbind /proc /mnt/build/proc
 mount --rbind /sys /mnt/build/sys
 mkdir /mnt/build/mnt/host
 mount --rbind / /mnt/build/mnt/host
 cp /etc/resolv.conf /mnt/build/etc/resolve.conf
 rsync /usr/portage /mnt/build/usr/ 
 rsync /etc/portage/make.conf /mnt/build/etc/portage/
 choort /mnt/build
 source /etc/profile
 ROOT=/mnt/host emerge -lv portage
 ROOT=/mnt/host emerge -lv system
 ROOT=/mnt/host emerge -lv world 

This handled most of the stuff, however it didn't pick up curl, and a few other pakcages.
I had to fight with installing some of the other stuff.
ROOT=/mnt/host emerge openrc curl boost

After this, on the shell of the host system, ran revdep-rebuild a few tiems
Once that was fixed I installed grub again, and built a new kernel.