Just for the record, I believe they're all 'flat screens', but they're not all 'panels'.
Flat screen Picture Tube TV's have been on the market for a number of years now. They are still considered Flat Screen TV's. The OP obviously means Panel TV's such as LCD or Plasma. Don't forget there are projection TV's too which can be 'Flat Screen'.
As for the changeover, that's to change to Digital broadcasts, replacing all Analog broadcasts. TV's will need to have a digital tuner to receive the broadcasts if they currently use their regular analog tuner.
This does not affect Satellite, Cable, your DVD/VCR hookups etc.
For those who do get their current TV over the air via an antenna, they will need to get the converters. They will cost about $40, but are basically free for those who request the $40 certificate from the government.
HD is digital, so anyone who is getting a HD signal over their outside antenna must already have a TV with a digital tuner, and in most cases have the UHF antenna since most HD stations are broadcasting at UHF frequencies and not VHF frequencies. Those receiving HD via cable or Satellite are not getting the signal from their TV's Antenna hookup, but either from their TV's CATV input, or HDMI input.