If you have a active account you can upgrade them both, the cable box should be free if is the one you're using as the main one (as long is not a HD or the new X1 box)and for you modem they charge between $ 10 to 15 more a month,but of course you can always talk to them and try to get some kind of deal(like i did).
If you have a current active account you should be able to upgrade the hardware. Unfortunately the new hardware will have a monthly rental fee that will show up on your bill every month.
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