The BaseApplicationPage code-behind file containing the BaseApplicationPage class and methods have been updated.
During migration, the existing BaseApplicationPage file in your application is replaced with new file. Any changes you have made to the BaseApplicationPage code-behind file will be lost during migration. This file is located in:
...\<Application Folder>\App_Code\Shared\BaseApplicationPage.cs (.vb)
After migration, apply any changes you may have made to the original BaseApplicationPage code-behind file to the new file.
Version 5.1 Application Migration