I have a requirement to migrate from Sharepoint 2013 Classic mode authentication to o365 claims based authentication.
But there is a catch. I need to do it one subsite at a time and not the entire application in one go.
There are different methods (MigrateUsersToClaims, deprecated MigrateUsers) which are called by your PowerShell-commands (Convert-SPWebApplication). All of these methods are scoped at a WebApplication Level.
With the given Information my answer is: It is not possible to convert a SiteCollection/SubSite/List/Item. You must do it at the WebApplication level.
Could you please provide more information abount why you need to migrate one subsite at a time? Maybe your doubts are unnecessary or we can provide a workaround for your problem.