It is not directly possible to go from PRTG 8 to 15. A partial migration (losing possible webinterface customization) is possible if you use PRTG 9 as intermediate step. Please contact email@example.com and about this. In that email, please also provide the last six digits of your license key.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.