[gentoo-user] HowTo rebuild everything same way differently

Hi,

ok, the subject: is something "helpless"... ;)

Before I screw up everything ... better ask:

My Gentoo system is kinda "old"...not in the sense
of not being updated regulary but in the sense of
"used for a (too?) long time"

Therefore I want to reinstall everything the same way (read: same
application and system funktionality) but in a different way: Using
a new stage3 (was it "3" ...dont remember exactly) and I want to make
the system "unstable" right from the beginning just to check, how that
works.

The only thing I dont know: How?

What is the best (sanest) way to acchieve what I want in the most
efficient (time) way?

Thank you very much in advance for any help!
Best regards,
Meino