This update, which is dated last week, doesn't actually let owners buy ebooks from Kobo but it does remove a few features. The link to the now disabled Sony Reader Store is gone, and so are the sharing options and the option for syncing bookmarks and annotations.
Unfortunately this update does not add any way to buy ebooks from Kobo, nor does it enable you to sync your existing Kobo ebook purchases. According to Sony's emails, those features will be coming in a later update which is expected to be released in late May 2014.
Just to be clear, this update is only intended for the North American market. Sony is still operating the Sony Reader Store in the rest of the world, and unless you want to be cut off you should not install this update.
Now, if you like the idea of tying your Sony Reader to Kobo, then perhaps you should consider the update. But even then I would suggest that you wait for the later update before installing anything.