The answer to your question is NO - what ever connection you were using prior to a flat panel will still work. Your flat panel will still receive the same signal, upscale it to it's higher display resolution, and hopefully it will look even better.

If you wish to receive HD content, then while there should be no reason why you need a DVR, or even a STB for basic service, you are pretty much stuck with whatever package your cable provider has decided you must purchase to receive the HD content.