Since yesterday you can download Oracle Database 19.3.0 on premises for Linux. And as of today, 19c for SPARC Solaris 64bit is available, too.
Oracle Database 19c on premises for SPARC Solaris
You can download Oracle Database 19.3.0 on-prem for SPARC Solaris 64bit now from the usual sources:
Oracle 19c Documentation
And of course you can access the Oracle 19c Documentation from now on as well:
See especially the operating system dependent install guides:
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Well, 


This question is coming up from time to time – and currently not covered in the official Oracle documentation as far as I know: “Can you simply switch from SE2 to EE with Oracle Multitenant?”