Hmmm, I posted a reply to this right before aorg maintenance apparently begun (is it over yet?), and now it's gone.
To summarize:
Software?
Aminet! The world's Number One Repository of old crud, WB screenshots and .mods!

"Boot to old OS"?
Depending on what that's supposed to mean:
A. Hold both mousebuttons at boot, choose to disable AGA and CPU caches!
B. Search for "degrader" on Aminet!
C. Buy a kickstart switcher!