Generally the licence key is built into the media, the only thing you have to say its licensed or not is a piece of paper saying I have X amount of licenses, then it is up to you to ensure that you are in compliance with that.

Now depending on your licensing agreement you may have downgrade rights, where you purchase SQL 2012 licenses but you actually install SQL 2008, but you need to ensure that you licenses the SQL 2008 box under the SQL 2012 rules, so per core not per processor. Check with your re-seller on if you have downgrade rights.

Evaluation edition will expire after 180 days, so you will need to ensure that you download the full edtion of SQL or ensure you have the full license key when installing from the evaluation edition media.

If the vendor of the application says its only compatable with SQL 2008, then I would heed their advice and leave it on SQL 2008 as they may be functionality which will break on the upgrade to 2012.

You could always run the upgrade advisor and see if anything will break, then upgrade your dev, test environment first and test the app out to ensure it works as it should in SQL 2012 before you upgrade your production box.

Compatability level doesnt make the database a SQL 2008 database. Once you upgrade from 2008 to 2012 the database changes to a SQL 2012 database, the compatabilty level just tells SQL that certain things behave differently.

As with all things licensing, don't take a public forums advice as the truth, always speak to your MS Reseller or MS directly