@Amit Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
Apologies for the delay response here!
So are you planning to create a new VNET and create ANF delegated subnet under it ?
Azure does not allow to change the subnet range as long it's still in use. (Once the volume is created, We can't change the volume name / IP address )
Workarounds : Creating a new vNet with vNet peering to the original to create new volumes there.
Please let us know if you have any further queries. I’m happy to assist you further.
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