When you open up Poser it will be the version that matches the serial number that you last entered. In your case Poser 11. Go to Help-> personalize and it will have your Poser 11 serial number. Replace that one with your new Poser Pro Serial number and now you will have the Pro version. It is best to deactivate your Poser 11 version first as it may cause issues. Plus by doing that you can install it again on another machine.

ETA Both Poser 11 and Poser Pro 11 use the same program, just the serial number is different which opens up the extra Pro features.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13