It is well known that SharePoint 2013 by default uses Claims mode for Authentication and Whenever we create a Web Application it creates with Claims Auth. SharePoint 2010 also had Claims but was optional for us to choose whether we want to have Claims based or Classic. If you had a Classic Mode Web Application in SharePoint 2010 and then it migrated to SharePoint 2013, you also need to Migrate Users specifically from Classic to Claims because you can no longer user Classic mode in 2013.
I had a scenario where an Intranet Site was Hosted in SP2010 and Migrated to 2013 which had hundreds of Libraries and Folders inside them with Unique Permissions. After the Migration from 2010 to 2013 using DB attached method, everything worked well until users start complaining that they are unable to see any records with unique permissions.
Checked the permissions using “Check Permissions” Option and something seemed to be wrong as all the users are not having their relevant set of permissions in the new Destination. It just the Read permissions granted through “All Users” Group which is NT All Authenticated Users.
Migrating Users from Classic to Claims using below PowerShell Script Rectified the issue and everyone was able to access their relevant records as expected. checked the permissions again and it appeared all set of permissions which is looked like everything back to normal. We also need to ask every individual to check their content and functions to verify this.
You can run this in any Server in the Farm and once it’s done, cannot be reverted back so better test it out first in a POC Setup. this will completely switch your all users from Classic mode to Claims.
Bottom Line: Claims Based Authentication is an Essential Component in order to enable SharePoint 2013 with Advanced functionalities. If you had a Classic mode Web Application in SharePoint 2010 and you are planning to Migrate it to SharePoint 2013, Migration of Users is a Critical part of the SharePoint Migration. It is important to test the same in a Test Environment prior to the Production.
Notes: There are lot more details and scenarios when it comes to Claims Authentication. will put it over here as an detailed article soon.