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Excerpts from Volodymyr Litovka's message of 2017-08-24 07:24:37 +0300:
> Hi Clint,
> 
> see inline, please.
> 
> On 8/24/17 2:21 AM, Clint Byrum wrote:
> > This is precisely the reason floating IPs that NAT to other IPs exists
> > (not, as we think, to provide public IP access... we can do that with
> > fixed IPs).
> >
> > Moving ports, moving the IP, they all involve a few layers of cache
> > invalidation and complex manipulation at the lower networking layers. But
> > changing a NAT destination is relatively instant.
> >
> > I'd recommend you using a floating IP for this. If you can't, please
> > explain.
> It's going to be public cloud and there can be few reasons to allow 
> customer to move pubic IP address between his VMs, e.g. he built another 
> VM using another OS for same role and need to move this role from old VM 
> to new VM, do not changing other infrastructure's configurations.
> 

That is precisely the use case for floating IPs, and doesn't preclude
doing exactly as I suggest.

That said, just taking the IP out of the pool, removing the old port,
and creating a new one with the IP as fixed_ip, will do it, albeit with
an unknown amount of downtime to due to ARP cache and perhaps other
caches.