Even though you have advised staff members a migration is taking place – you can guarantee someone will try to check-in or edit documents. The best way to prevent this is to put your content database into read-only mode, locking the content database.
There are two options to lock the content database.
Option 1 ( Recommended ):

Note: Read more at Manage the lock status for site collections in SharePoint 2013
Option 2 ( not recommended ):
NOTE: When some SharePoint timer services are run it may cause the site to display errors when the database is in read-only mode