That is true, the machine will retain its IP for the duration of the lease when the VLAN is changed. The trick I use to get around this is I shut the port down when I change the VLAN and then turn it back on again. For example:
Switch(config)# interface fa0/24 Switch(config-if)# shutdown Switch(config-if)# switchport access vlan 400 Switch(config-if)# no shutdown Switch(config-if)# end
The PC will see the network card reset and perform a new DHCP discovery. That saves you from having to go to the individual PCs.
Hope that helps,