If you have the old IP addresses already entered into DCR your best bet will be to export the DCR device list to CSV, modify the IP addresses in the file, then re-import the file overwriting what is in DCR. This way, all your device history will remain.
If, however, you do not have any IP addresses configured in DCR (i.e. just hostnames), and all of the hostnames will remain the same, all you need to do is restart dmgtd after the switchover.