Welcome to Microsoft Q&A Platform, thanks for posting your query here.
For moving the Azure stack HCI registration from one subscription to another is by Unregister and reregister.
This the way you can achieve your ask.
Ref: https://learn.microsoft.com/en-us/azure-stack/hci/deploy/register-with-azure
Similar Thread: https://learn.microsoft.com/en-us/answers/questions/598684/azure-stack-hci-subscription-delete.html
Hope that helps.
If you need further help on this, tag me in a comment.
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