It all depends ...
I assume you have the DHCP under your control and not
using our ISP's DHCP services.
Using someones elses ICANN assigned IP address range can
create problems like this. You should use one of the
private address spaces the IETF allow for internal network
use (not routeable directly across the I-net.)
The space you were thinking of using (Class C - 192.x.x.x)
makes sense but you need to go one octet lower to
192.168.x.x with a 255.255.255.0 mask to be within the
assigned private space.
Reconfiging DHCP table settings is quite easy depending on
the OS (system) you are using. You should do this of hours
and when you have access to all of the systems.
Now the rub ...
You will then need a Proxy or NAT firewall to connect to
the I-net as you will need some means to convert the
private address to a routable address that can cross the I-
net unblocked.
Working with your ISP or NSP you should be able to get a
pool of addresses you can use in your NAT firewall. It
maybe the ISP will just have you use there DHCP server for
this (not for your internal addresses). The other option
is to multihome our client systems. With two NICs then you
can have one for local resouce access (private address
space) and the second which uses your ISP's DHCP for I-net
connections. This assumes you have a broadband access
point that has a hub or switch connection the systems all
share (no router, proxy, or firewall present.) One
drawback here there is no security and your ISP may limit
how many systems can gain a DHCP address from them so at
times some systems may not be able to gain access to the I-
net at the same time others are. This method is messy and
does not scale well. For a very small office with limited
need to connect to the I-net it may make sense from a cost
perspective.
Using a NAT firewall is the best way. Get it up and
running first before you tackle the IP address change over
on all of the other systems (do one system manually to the
new address space when you setup the firewsll.) If you
have routers in your network you will need to subnet the
private Class C address space down to give your self
enough nets use a IP address calculator to figure out how
to devide the 254 station addresses you have to work with
here and what your growth is projected. Once you lock
down your addresses it can be hard to move the alocations
around if your short.
