The upgrading of toolchain elements (binutils, gcc, glibc) is rather significant. The amount of effort that goes into the upgrade depends on whether the package is a major or minor release.
For example, according to the Gentoo GCC Upgrade Guide , you may be able to get by with upgrading the package in question - if it was a rather minor release:
(Bold face emphasis mine)
Generally speaking, upgrades to bug fix releases, like from 3.3.5 to 3.3.6, should be quite safe -- just emerge new version, switch your system to use it and rebuild the only affected package, libtool. However, some GCC upgrades break binary compatibility; in such cases a rebuild of the affected packages (or even whole toolchain and system) might be required...
To be completely safe that your system is in a sane state, you must rebuild the toolchain and then world to make use of the new compiler.
However, if the change in the package is substantial, the guide states that the entire system would have to rebuilt.
Hope this helps.