During the Power BI Report Server installation, you will be asked for the Product key. You can find the key on VLSC at the description of you SQL Server 2016 EE license. There is a description about the license there.
After the installation, if you run the Report Server Configuration Manager you can check the 'Edition' to see if it is Evaluation or not.
Before you install it side by side with you actual Report Server, I recommend that you do it first in a testing environment.
You can also install it in a separate machine as Evaluation (180 days) migrate you SSRS and then upgrade the license running the setup again.
You can check this link for more tips about migration from SSRS to PBIRS https://powerbi.microsoft.com/en-us/documentation/reportserver-migrate-report-server/
Dont forget to have a backup of:
- You SSRS Database
- SSRS configuration files (rsreportserver.config, rssrvpolicy.config and web.config)
- SSRS Encription key