Why do you want to change this #? The COMx number is only there for compatibility reasons, only OLD applications will still use this number to "access" your modem.
Windows itself and pretty much any newer app that uses modems does so through Windows' RAS APIs. In fact, earlier versions of Rockwell's (now Conextant) winmodem drivers didn't even assign the modem a COM number.... The modem still worked fine however, just not in DOS or with old Windows 3.1 applications.
Winmodem drivers almost always setup the modem on Com5 or higher for one simple reason, to stay out of the way of devices that need to be assigned one of the standard COM numbers of 1 through 4.