Easy Answer:
You are already set, by using charter basic cable (no cable box) your channels will not change one bit.
For the confusing answer:
if you have any TV's that get their signal from an antenna, if your TV is NOT a digital set, (NTSC tuner only), come Feb 2009 you will need to go out and purchase a converter box from a retail store (best buy, CC, Wal-Mart, etc. You are also eligable for a government rebate). If you do not get a converter box, your set will not pick up any over the air (OTA) signals.
If your tv has a built in ATSC tuner, (IMPORTANT: SOME TV'S WERE SOLD AS HD-READY, THESE WILL STILL NEED A CONVERTER BOX) your OTA programming will most likely not change. I say most likely b/c the new digital signal will not always reach as far as the old NTSC OTA signal did.
Hope that helps!