Originally I asked about upgrading 2016 x86 to 2019 x64. But I couldn't wait longer for an adequate answer and upgraded to 2019 x86. So I changed this question for the next version that I'll call 202_.
Must I do anything to keep all my settings under File > Options (e.g. in Excel, Word, etc.. 2016), before I uninstall 2019 x86 and install 202_ x64?
Yes. You can't installed Office 2019 and Office 2016 on the same computer. In addition, you can't mix 64-bit and 32-bit versions of Office.
You can use the User State Migration Tool(USMT) to migrate Microsoft Office 2016 settings to Microsoft Office 2019. The User State Migration Tool (USMT) is included with the Windows Assessment and Deployment Kit (Windows ADK) for Windows 10. Download the Windows ADK from this website.
Before you uninstall Office 2016, you can use the tool to create a custom .xml file to migrate specific Office 2016 application settings. After installing Office 2019, you can use the tool to apply the gathered settings to Office 2019. For more information, read Migrate Application Settings.
If you uninstall Office before installing the new version, your Office files won't be deleted, but if your version of Office includes Outlook you may want to back up your Outlook data files. See Find and transfer Outlook data files from one computer to another.
There is a similar thread for your reference:How to migrate Microsoft Office settings?
