A line from the "Release Notes":
"Windows Defender (Beta 2) will uninstall Windows AntiSpyware."
...the rest of the release notes are on:
http://www.microsoft.com/athome/security/spyware/software/about/releasenotes.mspx
it's a fairly short read.
Defender appeares in the 'add remove' application and, if you really need
to, that's the place to uninstall it.
If you saved the Beta 1 executable to a download folder it should be easy
enough to reload it.
As I understant it, Defender replaces the Beta 1; I haven't seen anything on
how long Microsoft will continue to support the Beta 1.
Defender updates automaically through the Widows update site; you can check
your signature versions and, installtion dates for Defender at the Windows
update site.
Just click "review your update history"on the left of the site page.