> (Of course, one can always simply disable/re-enable extensions of questionable merit as needed until a determination has been made as to whether they have enough value to be worth keeping, and then uninstall those which fail to make the cut; disabling and uninstalling can both be done from within Safari's Extensions pref.)
That works for testing newly installed extensions, but not for bouncing back and forth between versions.
I just completed my experimentation and found that both installing an update via Safari > Prefs > Extensions and installing a d/l'ed extension overwrite both enabled and disabled extensions.
If you want to go to the trouble, though, the currently installed version of any extension (the .safariextz) can be found at /Users/Your short name/Library/Safari/Extensions.
(Another extension related gripe is that .safariextz files are invariably, in my experience, anyhow, distributed without version numbers.)
The new Great Equalizer is the SEND button.