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The AI-900 exam is retiring at the end of June. It's replacement AI-901 will be available from April in beta. The certification you earn, Azure AI Fundamentals, is not changing.
While there is some overlap between the exams, there have been a lot of changes to AI services especially Microsoft Foundry and this is the reason for the change. The Learn modules for AI-901 have been updated/rewritten and the focus is now on Foundry rather than individual AI services.
The AI-900T00 course is related to the contents of AI-900 exam and the AI-901T00 course is related to the contents of AI-901 exam. Some of the AI-900 Learn modules have been reused for AI-901.
So it depends on what you plan to do. You could continue with preparation for AI-900, or switch to AI-901.
If you are asking why the new Learning Paths are marked as completed, it will be because you have already been through the modules in the Learning Paths.