Some virtual com-port drivers seem to have a fixed com-port number. These drivers mainly use usbser.sys from Microsoft. You can change the inf-file of the device....
[USBtoSer.AddReg] ; AddReg section
The USBtoSer label can be different from your device. The last line is important. Please make sure that msports.dll is included in the inf file. After you have reinstalled the driver, you should be able to change the com-port number.