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To upgrade your workspace to a free Microsoft Fabric trial capacity, you can create a support request through the Microsoft Fabric support page. If you're an administrator of your Fabric tenant, you can create a support ticket in the admin portal. Unfortunately, there isn't a specific email address provided for contacting the Azure Fabric team directly.
If you encounter any issues while creating a support ticket, you can also explore the Azure Sales contact page for additional support options, including chat and phone support.
I hope these helps. Let me know if you have any further questions or need additional assistance.
Also if these answers your query, do click the "Upvote" and click "Accept the answer" of which might be beneficial to other community members reading this thread.