As someone in here mentioned, it is possible to add another line for $10/mo and then after activating that phone, transfer it your line, and put no phone/dumbphone on the second line. $240 + subsidized price for phone of choice.
I did the "upgrade on wife's line, activate, then throw her dumbphone back on and put the Lumia on my line" trick less than two weeks ago. Though the first store I was at refused to put the Lumia on my line. The second store had no problem with it, plus the girl helping me was cute. Bought some screen protectors basically for that reason.
The nice thing is that once you are on a 4G SIM, you can swap it between 4G phones without having to call in or swap numbers online or do the OTA programming.
Pay around $95/mo for our family plan including unlimited data and 250 texts on my line.
Also, FWIW, getting great battery life out of the Lumia. Averaging over 25 hours per charge. Pulled it off the charger last night at 10 pm and still at 35%, phone calls, texting, emailing, navigation, map lookup, browsing/searching, couple games.