Hello @Nathan Farrar
If you transfer an Azure Stack HCI product to an account in the same Azure AD tenant, there's no effect on the resources running in the subscription. However, if you're transferring to a different tenant, there are some steps you should follow to move. Your Azure Stack HCI account's system assigned, and user assigned managed identities will be cleared during the move to the new tenant.
This is because your Azure tenant houses all authentication information, so this needs to be updated for your Azure Stack HCI account in the new tenant. After the move, you should clear the old identities and create new ones.
If this is the first time you are doing this, I would recommend contacting the Azure technical support team for guidance.
If this does answer your question, please accept it as the answer as a token of appreciation.