Hey there!

As of now, there isn’t a way to change private IP addresses to a different subnet since they’re assigned to fit into the private network that is made up of all resources on your account within the same data center that have private networking enabled. That being said, I can certainly see the usefulness of this feature so I’d recommend finding an entry that includes your idea or creating an entry at https://ideas.digitalocean.com/ so that our team can get a sense of the community support behind this feature for prioritization.