Help changing TG585 v8 local network interface ip

[#1]

Hi
I am attempting to change my network's private ip address from 192.168.0.0 to the 172. . . range, but having limited success.
Would appreciate any pointers with the approach below, which ends up with me pressing the router 'factory reset' button because the PC running Windows 7 does not recognise the new lan ip as a private network,  does not pick up the new default gateway settings (ipconfig still has 192.168), and cannot access the router GUI using the new private lan ip, or the old one!
1. In router GUI, add DHCP pool in new range (see screenshot).  'Apply'. 
2. In router GUI, add new IP address (see screenshot).  'Apply')
3. In router GUI, delete 192. address.  'you will lose contact with your router?'  'Apply'
4. re-boot router.
5. re-boot PC
Many thanks
Paul

[#2]

I believe you need to disable DHCP before trying to change the LAN IP/subnet/DHCP pool. Obviously as soon as you do that you're likely to lose contact with your router.
So the procedure probably is something like:-
Disable DHCP
Set static IP on pc in 192.168.1.x subnet
Delete router 192.168.1.x DHCP pool
Change LAN IP of router to 172.x.x.x subnet
Set new static IP on PC in 172.x.x.x subnet
Add new router DHCP pool for 172.x.x.x
Reenble DHCP
Set PC back to DHCP and test

[#3]

Thanks MisterW - now sorted 
In case it helps anyone else, here's how it panned out for me:
Y: Disable DHCP
Y: Set static IP on pc in 192.168.1.x subnet
N: (no option for me to do this from the GUI - see below) Delete router 192.168.1.x DHCP pool
Y: Change Add LAN IP 172.x.x.x subnet to router
Here: Deleted IP 192.168.x.x ('you will lose your connection to the router.  Continue? Yes)
Y: Set new static IP on PC in 172.x.x.x subnet
Y: Add new router DHCP pool for 172.x.x.x
Y: Reenble DHCP
Here: Backup router configuration and, with Notepad++, edit out 192 DHCP pool and static IP address.  Import edited configuration back to router.
Y: Set PC back to DHCP and test (Hurrah!)
Many thanks for your help