You can buy SQL Server 2019 license and have installed a previous version, specially when you are upgrading to the newer version by components (SSIS, Data Warehouse, SSIS), on what is called a side-by-side migration.
In addition, for detailed information about the license issue, you can consult professionals in this area calling 1-800-426-9400, Monday through Friday, 6:00 A.M. to 6:00 P.M. (Pacific Time). For international customers, please use the https://www.microsoft.com/en-us/licensing/worldwide to find contact information in your locations. For Volume Licensing Service Support, please use the https://www.microsoft.com/licensing/servicecenter/Help/Contact.aspx to contact information for your country.