You can do this in the registry.
First, if you haven't already done this, bind the IP address to the machine in Control Panel | Network | Protocols | TCP/IP | Properties | Advanced.
Please go to IMail Administrator | localhost | Services. Stop All services and then close IMail Administrator.
Then run regedit. Go to:
If you see a key with the IP address you will using, open that IP address key and create a new String Value named Official. Set it to the Official Hostname of the domain you are converting. Then find the $Virtual00? key for the domain you are converting and delete it. Otherwise, rename the $Virtual00? key for the domain you are converting to the new IP address.
Open the key named with the Official Hostname of the domain. Change the Address value to match the new IP address. Close regedit.
Now, run IMail Administrator. Go to localhost | your-domain. Make sure that the Official Host Name is correct, and that the IP address is correct. Add any domain aliases (that should be set for this domain) to the Host Aliases field.
Restart the IMail services.