Thank you for all the replies and especially your wonderful bedside manner, mirayge.
I am paying near $150/mo for TV and Internet, and if there is something I can get without spending more, I don't think that makes me cheap. I am cheap, though, no doubt about that.
I think the first two replies have nailed it, the expanded basic is probably in QAM format and the flatscreen will decode it but the old TV will not, and neither will the converter boxes. I may check with the cable co. and see if they have the simple box that radiotvnut suggested. Good idea. We're already paying ten bucks a month for the fancy box.
Thanks y'all!