I have not made the change yet, but am considering it. I, too, just picked up an Option iCon 452 for about the same price ($122.50 shipped) off of eBay. I called T-Mobile and asked if I could convert one of my three lines to a WebConnect line and was told I could do it. The line I am considering changing is a $9.99 line with $5.99 T-Zones on it.
If you do get the $5 Preferred internet on your 3rd line, are you going to add the WebConnect line of service anyways?
I go back and forth on converting my 3rd line. I am hoping that the USB modem gets better service (as in higher speeds) than my TM506 that I currently am using to tether. Other than the slow speeds, I am very happy with the TM506 as a tethering device.