Migrating your Reporting Services instances to SQL Server 2012/2014 has a few key considerations.
First, the oldest version you can upgrade from is Reporting Services 2005 and that must be patched to SP4 (9.00.5000).
Next, any SQL Server 2005 report data sources are still supported in SQL Server 2012/2014. Here are the full list of data sources supported in SQL Server 2014 http://msdn.microsoft.com/en-us/library/ms159219.aspx. However your Report Server databases themselves can’t be hosted on SQL Server 2005 database instances http://support.microsoft.com/kb/2796721/en-nz.
Importantly, those pesky Report Models are depreciated in Reporting Services 2012/2014 http://msdn.microsoft.com/en-us/library/ms143509.aspx. They will migrate but you won’t be able to make changes after the migration – so at some point they will need to have their datasets rewritten. Getting a handle on how many reports hang off Report Models and giving your customers some options is a good way to manage this issue.
Finally, the full migration process is outlined here step by step http://msdn.microsoft.com/en-us/library/ms143724.aspx. There are risks in any major version migration, but running side by side environments and getting a business acceptance sign-off is the best way to mitigate those risks.