Sever Fault cannot answer licensing questions. Contact the vendor, in this case Microsoft or your reseller.
As of September 2019, the release notes for SQL Server 2019 explain it is in preview. With no support available, there is considerable risk should it not work for you in production.
SQL Server 2019 release candidate (RC)
SQL Server 2019 RC is the latest public release of SQL Server 2019.
SQL Server 2019 RC is available only as Evaluation Edition. No other
editions are available.
Complete details about support and licensing for release candidate
software are in license_Eval.rtf with your installation media.
Support
SQL Server 2019 RC is preview software. It is not publicly supported
for operations. Customers who are in the SQL Early Adopter
Program, may have support to run SQL Server 2019 RC by special
agreement, in consultation with Microsoft.
Regarding upgrade path:
While [side-by-side or in-place upgrade] from SQL Server 2008 and 2008
R2 is not blocked, there are no commonly supported Windows Operating
system versions between them and SQL Server 2019.
One upgrade strategy would be to build new test and production hosts on the latest Windows Server, 2019 if possible. This removes the need to upgrade Windows in place. If desired, run SQL 2019 previews in the test environment to understand it in advance of release.