BaseApplicationPage and BaseUserControlPage files replaced
The BaseApplicationPage and BaseUserControlPage files in your application’s App_Code\Shared folder are replaced with new versions from the ProjectTemplates folder.
Migration effects
During migration, the existing BaseApplicationPage and BaseUserControlPage files in your application are replaced with new files. Any changes you have made to the BaseApplicationPage and BaseUserControlPage files will be lost during migration.
What you should do
After migration, re-apply any customizations you may have made to the original files to the new files in your application.