Upgrading a complex infrastructure is not an easy task and it should not be executed without any preparation. Careful planning of the required steps will help you eliminate possible issues to the minimum and the upgrade will run smoothly. Through the versions of SQL Server, there have been many changes, so it's worth exploring your options while planning the new installation also from the perspective of the available edition.
While upgrading the SQL Server from older versions, you also have to understand the changes in the licensing for SQL Server where SQL Server 2008 R2 and older used the per-processor licensing model or user CAL licensing model. Starting with SQL Server 2012, Microsoft has moved to core-based licensing, where you need to have a license for each CPU core used on your operating system.