Currently, Azure Search does not support downgrading from a higher tier to a lower tier. You can only switch from a lower tier to a higher tier. To change to a lower tier, you would need to create a new search service at the desired tier and then deploy your search content onto the new service. After ensuring that the old service is no longer needed, you can delete it. Unfortunately, this means you would need to manage the costs associated with creating a new service.
Kindly refer below link: tier-changes
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