I have spoken to some colleagues about this just to be a 100% sure. Yes is the answer as you already have the trial installed.
To enable the upgrade, you will first need to process the upgrade in your account -
You will need to first process the upgrade to 2015 (you you do not have to install the 2015 software) then wait 30 minutes for our servers to sync and then repeat the process to Passolo 2016.
If you need further help please let me know.