I do not know if this is the appropriate forum or not for this issue, but please feel free to relocate this post if necessary.
I formerly had the Unity 1.5.1x web player installed (both in Firefox and Internet Explorer), yet needed the 1.6.1x player to view a demonstration. When I downloaded the new version, it would not allow me to simply overwrite the existing version of the web player–I had to manually uninstall it, which I see as being potentially inhibitive to easy use.
This could be fixed, yet a better option may be to run an authentication and auto-update when the plug-in runs.