There is the potential for a great deal to break. Upgrading libc is a significantly larger upgrade than, say, upgrading your kernel.
Luckily RPM lets you test things before doing them for good with the "--test" option. You'll be able to judge the extent of breakage by downloading the packages and trying to install them with "--test".
Also, keep in mind that future updates and upgrades may not work with a glibc other than the one provided.
But it strikes me that you might be asking the wrong question -- what are you trying to do that you believe requires a glibc upgrade?