Normally you have for example something like this:
Modes "1024x768" "800x600" "640x480"
Then you do "ctrl alt + or -" to switch virtual resolutions. For metamodes it is the same.
Real resolution switching is impossible at the moment, in the future it will work using something called RandR.
Likely you want to use your TV for movies. I thought there was a little hack you can use. If you do "1024x768@640x480, 640x480;" movie players will think 640x480 is the real resolution and will work fullscreen at 640x480.