Yoast Google Title Shows Current Site name appended with Old Site Name

Actually my web link is http://www.annauniverzity.com

My old site name is Anna Univerzity
My new site name is TNEA 2015

And In Yoast Titles and Metas Post Title Format is: %%title%% – %%sitename%%
For example in google searches
In some posts it shows as “Demo Title – TNEA 2015 – Anna Univerzity”
In Some posts it shows as “Demo Title – TNEA 2015”
In Some posts it shows as “Demo Title”
In most posts it shows as “Demo Title – Anna Univerzity”

But I need to show only “Demo Title – TNEA 2015” that is with new Site Name, So whenever i change the site name , google has to change all the sitename
I do no where i am doing mistake , i played around everything but still cant figure out