Office relocation, changing the new WAN IP address with new LAN IP addresses assigned by landlord.

[edhu25]

Hello, I urgently need help. My office is moving to a new location.
The internet connection is managed via the office landlord.
I received information about the internet connection..

I have been given a static public address and a LAN IP address with GW.
However, I need to keep the old LAN settings because I have multiple servers in this network.

Firewall is FortiGate 81F

new connection
Public IP: 87.xxx.xxx.xxx
LAN IP: 10.48.200.2/24
Gateway: 10.48.200.254

old connection on firewall
Public IP: 104.xxx.xxx.58
Gateway: 104.xxx.xxx.57
LAN IP: 192.168.2.1/24
no DHCP, all clients receive through Windwos DHCP, 192.168.2.1 is GW for clients

Is it possible to use the new internet connection with the existing LAN 192.168.2.0/24?
Do I have to apply the new public IP Address for WAN and LAN settings for internal,
-> setup route interface WAN to 10.48.200.254
-> and then set up another physical interface with LAN 192.168.2.0/24
-> set up interface 192.168.2.0/24 route to 10.48.200.254

Unfortunately I can't test it in advance but directly during the move

Is this the right way? Or how can it be solved and what steps are necessary.
I am grateful for any help.

[sw2090]

the WAN IP will have to be set I guess (unless there is dhcp)

You could keep your existing wan by NATting all Internet traffic with the new lan ip.

[edhu25]

I get this additional information:
Public IP: 87.xxx.xxx.xxx
LAN IP: 10.48.200.2/24
Gateway: 10.48.200.254
Logical Ports: 1:1 Nat

Do I understand it right, that I need to setup 10.48.200.2 as WAN address and 10.48.200.254 as GW? And then the WAN address will be 87.xxx.xxx.xxx trhough NAT?
And I can keep the 192.168.2.1/24 as LAN IP Address?

[Toshi_Esumi]

Ask the landload if that setting is for the ISP's router's or your router's. If you can't get a proper answer, you probably need to bring in a laptop and a simple/cheap router to hook up wherever you are supposed to hook up your network prior to the moving-in date to see if different LAN subnet w/ NAT wouldn't work. 
If restricted/filtered you probably can ask the ISP directly to change their side settings.

Toshi