Hi @Drew , welcome to Microsoft Q&A forum.
Unfortunately we cannot move the VNET or Subnet after the deployment of Azure SQL Managed Instance. Below articles mention the same:
Configure an existing virtual network for Azure SQL Managed Instance
Can I move my managed instance to another subnet?
However, the same article mentions that we can create a new instance with new VNET and Subnet and move the data from older instance.
Below is the article that talks above restoring the backup between different Azure SQL Managed Instance:
Restore a database in Azure SQL Managed Instance to a previous point in time
Please let us know if this helps or else we can discuss further on the same.
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