I've created the NIOS for a Project in Quartus Pro 19.3.
How can I migrate the generated NIOS to another Project (also maintained under Quartus Pro v19.3)?
You can copy the generated Nios to another project folder. In Platform Designer, click Refresh, and your generated Nios will appear under System, and then you can add it in your top level.
You can maintain your Quartus Pro v19.3 in Platform Designer without upgrading.