We've had HD Cable & an HD Box from BrightHouse ( TWC Affiliate ) for three years now.
For your TV to work full screen you need to be able to feed it with either HDMI or Discrete Video ( The RCA style Red Green & Blue cables. ) If your current non-HD cable box supports the Discrete Video you should be able to cable it up and get a full screen with minimal distortion, we were able to do that with our old non-HD box when we first moved to Florida.
BrightHouse has offices and does do exchanges of equipment, but I'm sure that TWC would want to send a tech out to your place to setup a new HD Box. Should only take them 30 minutes tops. All the ones we've had out ( Our Internet is RoadRunner and has had issues with upstream equipment big time. ) were courteous and efficient.
I received compliments on my Anime Posters, Managa Collection & Model Railroad layout. The Techs are geeks too.
Happy belated anniversary to you and your wife, mgosdin!! ^ω^
Yesterday was also a pretty important day for me. :3 It was the last day I ever have to attend High School, and thus I am now on my way to college. I still have to go through the graduation ceremony (which may be when it hits me I will not see many of the people I know again), but as far as I am concerned it is all done. ^_^
For you my friend the fun is only just beginning, trust me on this.