"Today we are sharing our plan to automatically upgrade Windows customers to the latest version of Internet Explorer available for their PC." First sentence of the article seems pretty clear to me, the latest IE that your PC can run will be installed. Not sure why the confusion.
I wonder why they decided to be so non-specific in this article. It would have been more clear to say "Users on XP will be upgraded to IE 8, users on Vista and 7 will be upgraded to IE 9 and later as new versions are released" or something of the sort. Instead, they spent a lot of time talking about why what they're doing is so great, and a lot less time talking about exactly what they are doing.