For a set number of ranked rounds after tiering up, your average WILL GO up. After said number of ranked rounds are played, then it will start to go back down and not up. I believe it was Ms. Lizzie who informed me when I made Tour Master, that 50 ranked rounds were needed to get the average to stop going up. If i'm not mistaken it was then said that upon reaching Legend, there is 400 ranked rounds needed to stop it from going up. This is just my experience.
Oh, and congrats for making Pro :)