You actually make this sound really complicated but to be honest from my perspective its easy. Give your app server a new physical ip address which is obviously static. Run the configuration wizard. Then configure the HA VIP with the old physical ip address of the server. Just keep all on the same subnet. That way the polling address will be the same in a HA situation.
Hi MathieuJM like James says try not to over think it, the documentation doesn't make it as clear as it should.
Basically, if you want to keep the current IP to use as the VIP, then I would suggest you assign 2 x New IP's to use for the Primary and Standby Server. I would recommend they are HIGHER than z.x.y.132 (.132-.254) to ensure that the VIP IP is the closest match to the Gateway.
Then as mentioned just change the existing servers IP and run the Configuration Wizard, this will update the IP in the database, then you will be fine to deploy HA.
Thank you all.
Due to this particular configuration we finally choose not to keep the current server IP address as the VIP. We will choose a new IP address that is before the .128.
with a gateway ip between .1 and .127 we need to choose a VIP in the same half range IP.