Replace Classic Root Site Collection with a Modern Site?

Hi Christophe Barneaud ,

this depends on what exactly you mean by "all subsites , Teams sites that I have under my root site"

If you mean all subsites in the Root Site Collection (basically all paths that don't start with "https://yourtenant.sharepoint/sites/" and "https://yourtenant.sharepoint/teams/"), then these sites will be moved as subsites to the new destination.

If you mean all sitecollections apart from the Root Site Collection (like the SharePoint Sites behind teams or yammer groups), so basically all Urls that start with either the path "https://yourtenant.sharepoint/sites/" or "https://yourtenant.sharepoint/teams/", then they stay as they are.