Upgrading SQL Server 2019 from CTP to RTM version
I cannot find anything specific to SQL Server 2019, but I was able to find unofficial references about previous versions of SQL Server not supporting CTP to RTM upgrades.  Additionally, the reference Bahtiyar posted in the comments also shows that upgrades from previous versions of 2019 are not included.  Once the RTM is available, the documentation for the CTP builds is gone, so I'm not sure if this limitation is included in the documentation of the pre-RTM releases.

This makes sense from the standpoint that the preview and release candidate versions are not licensed, supported versions.  Supporting upgrades from these versions would require Microsoft to spend a lot of additional resources in testing those upgrades.  Additionally, this might encourage customers to put a production workload on a CTP or release candidate build thinking they can just upgrade to RTM, so by not supporting that upgrade, they are discouraging customers from putting production workloads on unsupported versions.