Changing TG585 router IP address and DHCP pool

[#1]

My Thomson TG585 v8 Router is set up by default to use 192.168.1.254 as gateway with DHCP pool 192.168.1.64 to 192.168.1.253. I would like to change the 1 to a 6 to avoid conflicts when using a VPN.
I have found some instructions on this forum  referring to a website which are helpful (http://www.speedtouch.net.nz/500ChangeIPaddress_v4.3.htm) but they are changing the pool to the 10.0.0.n group where 10.0.0.138/24, is already set up as an IP Address/Mask in the router configuration.
To achieve my aim do I need to also "Add" IP Address/Mask 192.168.6.254/24 in the LocalNetwork Configuration page?
Also, the instructions to which I refer use all the addresses of 10.0.0.n with a Subnet mask of 255.0.0.0. If I use a gateway of 192.168.6.254 and DHCP Pool covering 192.168.1.64 to 253 is the mask still 255.0.0.0 and if not, what should it be. If easier I would be quite happy to have gateway as 192.168.6.1 and allow all addresses from 192.168.6.2 to 192.168.6.254 in the DHCP pool when, presumably, the mask would remain as 255.0.0.0 .
I have been trying to achieve this change for some months and on every occasion have needed to do a factory reset on the router to recover so would really appreciate some advice.
I did post this as a question in support, thinking it was a simple question only to be told that PlusNet do not support this!      They did, however, refer me to this forum. 
For interest, I am using firmware v7 on the router and have no upload speed problem on my netbook running W7 Starter.  It seems that there is no reason to upgrade the firmware to v8
Thanks

[#2]

Quote from: inspiredron

the instructions to which I refer use all the addresses of 10.0.0.n with a Subnet mask of 255.0.0.0.

I think that subnet mask is for all the addresses 10.x.y.z.  You need to keep the standard subnet mask of 255.255.255.0 then just change the 1 to a 6 in the gateway address and DHCP pool limits.

[#3]

But do the DHCP range first then the gateway address. The DHCP change does not cause a reboot but the gateway change will. If you do it the wrong way around and your PCs are using DHCP you won't get anywhere. Doing the DHCP first means when the router reboots after the gateway change your PC will be given the new IP range and be able to contact the router again.

[#4]

I'm a bit confused. 
What I have done so far is shown in the attached screen shot jpg which shows the configuration screens for Adding IP and DHCP side by side
I clicked "Add" in the bottom half of the right hand window (which did not have LAN_Private) which brought up the left window. 
Entering the values shown in the left hand window and clicking "Apply" put the LAN_Private line in the right window as shown. 
I have not yet deleted all the devices so that I get the "delete" option against the default LAN_private DHCP pool.
Should I "Add" 192.168.6.0 to 192.168.6.254 (which is I believe 192.168.6.254/24in the top part of the right hand window before I delete LAN_private and hit "Apply" in the right hand window?
Peter - I don't seem to have the option to do Gateway and pool separately as they are in the same configuration input screen.
Reed - I thought 255.255.255.0 but 255.0.0.0 seems to have worked - or will it make all of 192.x.y.z, appear to be on the local network when only 192.168.6.z should be local?

[#5]

Quote from: inspiredron

Reed - I thought 255.255.255.0 but 255.0.0.0 seems to have worked - or will it make all of 192.x.y.z, appear to be on the local network when only 192.168.6.z should be local?

I'm by no means an expert on this topic but AFAIK the answer to the question is yes.

[#6]

Quote from: ReedRichards

I'm by no means an expert on this topic but AFAIK the answer to the question is yes.

Thanks, Reed.  Yes, I realised after I had posted that the whole of 10.x.y.z is reserved for private networks - hence 255.0.0.0 to allow anything in that huge range of IP's .  As you said, the mask should be 255.255.255.0 to cover just 192.168.6.n .  Those web instructions have actually used a rather bad example change, particularly as the IP range was left in the router default configuration.