To move an old project into TIA Portal (WinCC Basic/Comfort/Advanced), you often need to first open and save the project in a specific version of WinCC flexible to ensure the migration tool recognizes the file format.
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For modern operating systems (Windows 10/11), it is recommended to migrate WinCC flexible projects to WinCC Advanced in TIA Portal Key Considerations for 2007 Version: OS Compatibility: To move an old project into TIA Portal