Also, to avoid too many surprises, you might find it helpful to subscribe to one of the mailing lists like (WARNING: ~100 mails a day on this list) since pretty much any major update issues are likely to show up on the list with plenty of discussion explaining how to solve them.
If you were unlucky enough to notice some broken packages after the update then you would be sure that they aren't a result of removing any loose dependencies.
Multiple versions of GCC can be installed side-by-side in a Gentoo GNU/Linux system.
In the last couple years I've become much more careful about wholesale updating, electing instead to do it piecemeal after viewing the update tree.
Gentoo development seems to have passed its heyday. Anyway, one additional thing you might try, just to be really fastidious about the whole process, is cleaning out your distfiles. emerge --sync //Synchronize the package manager with the latest //version of each package.
In the case of GCC 4.7, the ABI Change was the adoption of the new C 11 Standard, which peterph also pointed out. At the time, all those tags increased confusion, and IMO made work harder.