@Somiya Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
Changing the account access tier applies to all access tier inferred objects stored in the account that don't have an explicit tier set. If you toggle the account tier from hot to cool, you'll be charged for write operations (per 10,000) for all blobs without a set tier in GPv2 accounts only. There's no charge for this change in Blob Storage accounts. You'll be charged for both read operations (per 10,000) and data retrieval (per GB) if you toggle from cool to hot in Blob Storage or GPv2 accounts.
Only hot and cool access tiers can be set as the default account access tier. Archive can only be set at the object level. On blob upload, you can specify the access tier of your choice to be hot, cool, or archive regardless of the default account tier. This functionality allows you to write data directly into the archive tier to realize cost-savings from the moment you create data in blob storage.
Can I change the storage tier of my Blob storage account?
Yes. You will be able to change the storage tier by setting the 'Access Tier' attribute on the storage account. Changing the storage tier will apply to all objects stored in the account. Change the storage tier from hot to cool will not incur any charges, while changing from cool to hot will incur a per GB cost for reading all the data in the account.
When a blob is uploaded or moved between tiers, it is charged at the corresponding rate immediately upon upload or tier change.
When a blob is moved to a cooler tier (hot->cool, hot->archive, or cool->archive), the operation is billed as a write operation to the destination tier, where the write operation (per 10,000) and data write (per GB) charges of the destination tier apply.
When a blob is moved to a warmer tier (archive->cool, archive->hot, or cool->hot), the operation is billed as a read from the source tier, where the read operation (per 10,000) and data retrieval (per GB) charges of the source tier apply. Early deletion charges for any blob moved out of the cool or archive tier may apply as well. Rehydrating data from the archive tier takes time and data will be charged archive prices until the data is restored online and the blob tier changes to hot or cool.
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
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@Somiya Changing the default does incur a charge (see the paragraph here): Access tiers for Azure Blob Storage - hot, cool, and archive | Microsoft Learn
To estimate the cost, just need an object and GB count (if going from cool to hot).
If you still find any difficulties in pricing, for more detailed assistance on this kindly contact Azure billing and subscription management it's free, and it's the best choice for you and they could provide their expert guidance on your scenario