The change was announced in the Microsoft 365 Roadmap ID 57131 (https://www.microsoft.com/microsoft-365/roadmap?filters=&searchterms=57131).

Classic SharePoint Office 365 Team Sites will be switched to use the Microsoft Search search box as in modern sites (at the top of the page in the suite navigation bar).

Search experience in Classic pages before the change:

Search experience in Classic pages after the change:

The change will be NOT applied to any team sites where the Classic publishing feature is turned on, custom result types exist, or complex query rules on the default result source has been defined.

If you do NOT want that Microsoft switch to the new search experience, use the following PowerShell commands, logging in as the site or site collection owner:

- Use PnP PowerShell to connect to the Site Collection:

    Connect-PnPOnline -Url [SiteCollUrl] -UseWebLogin

- Stay with classic search experience for the current site:

    Set-PnPSearchSettings -Scope Web -SearchBoxInNavBar ModernOnly

- Stay with classic search experience for all sites in the whole Site Collection:

    Set-PnPSearchSettings -Scope Site -SearchBoxInNavBar ModernOnly

If you do WANT that Microsoft switch to the new search experience (for pages that meet the conditions enumerated before, for example), use the following PowerShell commands, logging in as the site or site collection owner. Modern search can be applied only to Team Site or Publishing Site that use template ids that contain "STS", "CMSPUBLISHING", "BLANKINTERNET" and "GROUP":

- Use PnP PowerShell to connect to the Site Collection:

    Connect-PnPOnline -Url [SiteCollUrl] -UseWebLogin

- Change to modern search experience for the current site:

    Set-PnPSearchSettings -Scope Web -SearchBoxInNavBar AllPages

- Change to modern search experience for all sites in the whole Site Collection:

    Set-PnPSearchSettings -Scope Site -SearchBoxInNavBar AllPages