Sabrin Alsahsah migrates a SQL managed instance:
We have recently received a few service requests from our customers to know how to move an Azure SQL-managed instance between subscriptions. In this blog article, we will be discussing supportability, methods, and solutions that can achieve this goal, and you can decide the best for you depending on your business requirements and environment setup.
Currently, at this time there is no direct way to move a managed instance itself, but we can have a workaround by moving the Azure SQL managed databases across Azure SQL managed instances that are hosted\located in different subscriptions.
Click through for three workaround options.