The update that was released this week will update Premiere from 9.02 to 9.1. This new version will wipe out your old one. I don't believe it's possible to have both versions running on your machine (at least I haven't figured out how.)
This means every project you open that was made on 9.0 will now have to be re-saved to this new version.
Correct. Excerpt from Premiere Pro blog:
Please note that the 2015.1 version of Premiere Pro will overwrite your 2015.0 installation. Also important to note is that 2015.1 requires a revision to the project format and as such projects saved in 2015.1 are not compatible with prior versions of Premiere Pro CC.