The ßeta and older versions of Safari cannot be on the same boot volume (actually any mounted volume, I think). The OS apparently will get confused as to which app to run.
Actually not illogical, since two versions of the same app, with identical names would confuse most any OS.
But the Safari ßeta doesn't
exactly overwrite the older version(s), but it does move them to a hidden location. It still uses the old prefs and most other ancillary files, AFAIK. The ßeta Installer comes with an
UNinstaller that basically removes the ßeta and restores the old version to visibility and usefulness. I didn't know this, but it didn't matter since I just deleted the new version and uncompressed the old one. That's an old habit which worked in this case, but I can't guarantee all cases.
The point is, when an Installer has an
UNinstall option (often hidden, check all the buttons/tabs/popup menus) it might be a good idea to save that Installer, just in case.