He has a point there. The question says you've upgraded your server, and it's a database which already exists. The upgrade process will leave this dabase's compatability level at 110 specifically in order to avoid any surprises from the new cardinality estimator, and you are advised (on the page referenced as well as in SQL Server 2014’s new cardinality estimator (Part 1)) to test thoroughly before using the new cardinality estimator for an old database in production. So the correct answer to this question is actually "do nothing - you'll get the old cardinality estimator for your old databases anyway".

edit: at least the question says you aren't changing any code in that database, which seems to me to imply it already exists.