On 10.1.0 application, the tooltip of close button on Quick Selector pages and add record pop-up pages come from resource files. In addition, the style of email master page is changes. As a result, Email and Popup master pages are updated during migration.
During migration, the Email and Popup master pages are replaced with updated versions.
After migration, re-apply any customizations you may have made to the original files.
Version 10.1.0 Application Migration