You want to change the version that glibc reports to other programs?
Definitely a bad bad idea (grepping the sources for something like version might shed some light, or even better for the current version string).
I'm probably misunderstanding your question, but if I'm not, why not just compile the new version? To change the version string you'd need a recompile anyway, and that would probably break every program you try to download, because it would be looking at a version of libc that has one API but a different #.
Why do you want to do this?