It will not automatically convert the users in the content database from classic to claims when attaching a content database from classic web application to claims-enabled web application.
So you could migrate the valid users from classic to claims-enabled by the command below.
$wa = Get-SPWebApplication http://webAppUrl
And some site collection administrators deleted from Active Directory also will not be converted automatically.
So you could identify the site collection administrators by the command below, then delete them from site collection.
$web = Get-SPWeb http://webAppUrl/sites/rootWebUrl;
For more detailed information, you could refer to the article below.
Test-SPContentDatabase Classic to Claims Conversion
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.