I think there are some restrictions when changing subnets on Azure as follows.
[MS documentation excerpt
If there are no resources deployed in the subnet, you can change the address range.
If the resource exists in a subnet, you must first move it to another subnet or remove it from the subnet.
You must first move the resource to another subnet or remove it from the subnet.
If the resources in the subnet are being moved to the DR by ASR, is it possible to support range expansion?