In a recent thread, Jeff Wright (Adobe staff) said that Acrobat Standard and Pro installers are two different files.
http://forums.adobe.com/message/5071563
Apparently the Pro installer will not accept the Standard version serial number.
Since there is no Standard download file available at prodesigntools, no answer was given as to where customers can now download previous versions of Acrobat Standard. So we're left in the dark on that point.