The Header control is replaced with an updated version.
The Header.html file in your application's Header and Footer folder is automatically replaced with the latest version from the current design theme folder.
The Header.xml, Header.ascx, and Header.ascx.cs (.vb) files are deleted and will be automatically re-created when you next build your application.
Customizations you have made to the Header files will not be retained.
After migration, re-apply any customizations you may have made to the original Header files to the new Header files in your application.
Version 6.0 Application Migration