Upgrade to CRM 2015 from CRM 2013 but using Windows Server 2008R2 and SQL 2008 R2

Hi All

Just have a question regarding our existing CRM 2013 environment as we begin to plan to move to 2015 or even possibly 2016 depending on when we move... Likely to be in the next 12 months. 

I understand that an upgrade from CRM2013 with SP1 to CRM 2015 is possible but am I right in assuming this is only possible if the supported OS and supported SQL is already present on CRM 2013?

Our on premise CRM 2013 is based on Windows Server 2008 R2 and SQL server 2008 R2, so I am guessing it wont be supported so would need to do a fresh install. 

Before I go to management on this, any advice on this would be most appreciated.

Thanks