Unfortunately, you cannot preserve a Droplet’s IP address when transferring the Droplet between regions, nor can you reassign a non-reserved IP address to another Droplet. See our article about how we manage IPv4 addresses for more information.
If you need an IP address that you can reassign to different Droplets as needed, use a Reserved IP address. Reserved IP addresses allow you to retain an IPv4 address and assign it to resources as needed.