Cable HD TV Systems

I heard somewhere that signals used for for HD TV are 
incompatibile with all existing TV sets.
I also heard that there is a plan circulating to "transition"
both broadcast and cable sytems over the next 5-10 years
to 100% HD TV..

In effect, this plan would obsolete, virtually "over night",
the hundreds of millions of TV sets in use for the last 50 years.

Here is a source I found For info on the different standards used.
http://www.ivchd.com/DefTVstandards.htm

United States broadcasters tested various approaches to HD 16:9 for many years. 
Limited HDbroadcasting started in the U.S. in late 1998, 
using a digital HD system with 1080i (interlace) lines of active picture, 
followed by another digital system with 720p (progressive) lines. 

HD programming is now broadcast several hours a day in many markets 
and is available by satellite on a rapidly expanding schedule. 

My questions.
1. Are any converter boxes on the market today capable of
converting these signals to Standard Definition signals?

STANDARD DEFINITION (SD) BROADCAST SYSTEMS 
may either be analog (NTSC / PAL / PAL-M / SECAM) or 
digital (DTV), having 625 or less scan lines and an aspect ratio of either 4:3 or 
16:9.  
By 2006, most SD broadcasting will be digital. 

2. Apparently the "over the air" broadcaters will join the "digital" age.

Is it legal for me to attach such a device to my antenna, and cable,
and continue to use the TVs I now own..

3. Will I have to "steal" services coming into my home,
 to use the perfectly adequate TVs I presently own?

What do the "fine print" experts have to say about this.

--
Ray