Comment 16 for bug 1534374

#14 - I'm honestly not sure; I expected dependent packages to complain about the downgrade, but they didn't.

(Note: It's likely that all relevant architectures must be installed with the same dpkg command. Maybe it'll work if you use --force-depends?)

This worked for me:

$ sudo dpkg -i gcc-4.9-base_4.9.2-10ubuntu13_amd64.deb gcc-4.9-base_4.9.2-10ubuntu13_i386.deb
$ sudo do-release-upgrade