Fortigate works, how to assign IP/Netmasl in use by another device

Hi Folks,

This question is super easy.  Sadly, I've used a few $ervice provider$ and I have to learn this stuff on my own.

I have two devices - a Fortigate 60F and now a Fortiswitch 108E.  

On the Fortigate, I've configured port 'b' to 10.30.10.0/255.255.255.0 for example.  On port 'b' of the Fortigate, I have a cheap managed switch that works.  My computers all have manually configured IPs and they are working.   The problem is that traffic is not being managed by my cheap switch, and the solution is a better switch.

The problem: How do I change the IP/Netmask from port 'b' to the Fortiswitch on port 'a'?

So far, I have the Fortiswitch registered in Forticloud, the Fortigate recognizes the switch and I have configured a 'FortiLink Interface' with an IP/Netmask of 10.31.10.1.255.255.255.0 on port 'a'. The problem is that i don't know how to make the switch 10.30.10.0.  I understandably get the error Conflicts with '10.30.10.0-LAN'.  

Thank you,

TFPJ