Stupid MS Office Q: "converting" from old ISO install to new O365 (click-to-run?) and keeping user prefs?

Hey r/sysadmin . I thought about asking this elsewhere but I know this community is huge so figured it might be a good spot.

5+ years ago I was deep into MS administration, but I've been working with Macs and Linux since 2020 so I don't trust my own knowledge, hence asking here.

I've got a single system with an old (out of support) copy of Office2016 (not the nonexistent "2017" I said originally 😆). It was a from-disc / ISO install. The user has customized a \*\*lot\*\* of stuff about the installation - tweaks to the buttons above the Ribbon in Excel being the largest thing, but also custom normal.dot for Word and whatever the Excel equivalent is too. User is an accountant/CPA.

I want to get them onto a modern copy of Office, but when I download the installer for O365 and run it, it complains that it can't upgrade in place and that I have to uninstall the current copy in order to install the O365 version. If I do this, will the user lose any of their settings / tweaks, or will it all stay as-is?

\*\*TLDR: does uninstalling an ISO version of Office screw up any customized settings, or will they all stay as-is and work when a click-to-run copy is installed immediately afterwards?\*\*