Hi David,
Thanks for posting in the community. We are happy to help you.
Regarding your query, this change will impact all users. To my knowledge, there is no such built-in feature/method that can only affect specific users in SharePoint, however, as it also is a team for Microsoft Teams, you can achieve your needs in the Teams client. The "File open preference" in the Teams client only affects individuals.
- In the Teams client, click the three points beside your avatar, Settings.
- click the "Files and links" tab, under "File open preference", choose "Desktop app". After that, when you click the file in Teams, it will automatically open the file in the Office desktop app by default. The "File open preference" in the Teams client only affects individuals.
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If you don't see the option "Desktop app", please note that it requires some prerequisite points, such as the option "Desktop app" in "File open preference" is only available for Office 365 E3 or E5 subscribers. For more details, please view this article.
Appreciate your understanding and patience.
Sincerely,
George | Microsoft Community Moderator