EVERYONE is making this out to be WAY harder than it needs to be.. I've done this dozens of times..
NO, you dont even need to worry about VLANs or any switch configurations... First, let me say that I'm going to assume 2 things in my recommendation, just for the ability to tell you how to configure specific things, but other manufacturers can do the same thing, you just need to know how to configure it... let me know if my assumptions are wrong, and I'll try to clarify.. First, I'm assuming a Cisco router, so my config specifics will show details from a cisco config..
Second, I'm assuming you're using a Windows server for DHCP.. Win2000 or later..
So, Microsoft has something called a "SuperScope" when it comes to DHCP.. It is EXACTLY for this purpose, and it will assign addresses from both subnets to one lan segment/vlan.. Technically, if you want to do a "flash"/overnight cut, you wont even need to do this, but to make it smooth, you may as well use it..
Here's what you need to do..
1: Setup your Cisco router to think that both networks are on the same VLAN/subnet... For example if your config looks like this:
Interface FastEthernet 1/0
ip address 192.168.1.1 255.255.255.0
Then, you would add a "secondary" address to that interface, like this:
Interface FastEthernet 1/0
ip address 192.168.1.1 255.255.255.0
ip address 192.168.26.1 255.255.255.0 secondary
This will make the router answer as both 192.168.1.1 as it always has, but also as 192.168.26.1 for the new subnet.. It will route between the two subnets as required. At this point, to prove the config and satisfy your curiosity, manually configure one of your workstations as 192.168.26.2 with a gateway of 192.168.26.1, using your same old DNS servers, and you should have full connectivity without messing with anything else, including VLANs, etc..
NOW.. On your MS DHCP server, create a new scope for the 192.168.26.0 subnet as if it were going to be a separate subnet... use 192.168.26.1 as the router, and for now, use the same old DNS servers, or better yet, include the old addresses for DNS, AND the new addresses for the DNS servers once they're moved (ie: if your DNS is currently 192.168.1.5, put that in, and also put 192.168.26.5, so that the workstations will try both addresses - it'll make migration that much more seamless)
NOW, on the DHCP MMC, right-click on the server, and select "New SuperScope"
Create a superscope with the wizard, and put both the 1.x and 26.x scopes into the superscope. This will allow the server to begin giving out 26 addresses if there are no more 1.x addresses available, and lets the server know that those 2 subnets are really on the same VLAN.
Next, create an exclusion of ALL of your 1.x addresses, so that the server wont let any 1.x addresses renew, and it will force all your users to the 26.x subnet.
Once all your workstations have selected their new addresses, manually change your static server addresses to 26.x, and MAKE SURE DNS GETS UPDATED.. delete all the old 1.x addresses in DNS if they dont go away themselves...
At this point, everyone should have the new address, and you can remove the superscope, remove the 1.x scope, and make the Cisco router interface have only the 26.1 address...
This should be easy, cause no downtime, and you dont have to worry about any VLANs..
Here's some links:
details on Superscopes:
http://technet.microsoft.com/en-us/library/cc757614(WS.10).aspxCreating superscopes:
http://technet.microsoft.com/en-us/library/cc784138(WS.10).aspxLet me know if you have any questions..
-Steve
