The new license is for the Pro theme which you directly purchased from themeco. But the license which you purchased in 2015 is most probably purchased n Themeforest.
You need to go to your ThemeForest account and copy the license per the instructions here, then you need to go to your Themeco licenses page and click on Register License button:
And paste the code you copied from Themefores:
Then you will have the licenses added to the Themeco. Then you can validate your installation reading the guide my colleague already gave.