Firstly create a new test vlan with an IP subnet on the DHCP server and make sure you can get an IP address in the sites because acls etc. can make a difference.
Once you are happy that DHCP to the server is working the easiest way is to notify all users to shut down their machines after work one day and then you add the "ip helper-address ..." commands to the SVIs on the switches and remove the local DHCP pools.
That is by far the easiest way and why it is important to test first because you don't want users coming in next day logging on and not getting an IP address.
And obviously you need to be there before anyone else the next day just in case.
One of the joys of our jobs :-)
Jon
