Enter Volume Licence Code into Evaluation Copy

Hi, Im a complete novice with SQL, I have never used it but had to install it on a 2012 Essential Server for one of my customers. I installed the Evaluation version which I downloaded but it has now expired, I was able to get the volume licence product key from the 2014 Standard version that the customer has purchased. I have run the SQL 2014 Installation Centre and then Maintenance and Edition Upgrade and entered the product key but I get an error saying there are validation errors on the page. Can anyone tell me if I am able to enter the purchased product key on the evaluation version and if I am doing it the proper way. Thanks Damien