Hello,

Anyone knows how to change HDTV to SDTV at IPTVserver to IPTVclients, it is because I want to lower the traffic of the network cause by receive HDTV.

Current version not support transcoding for h264. I hope in near future.
Also for h264 -> SD you must have hi-end CPU. May be hi-end CPU for every channel.

HDTV receivers can also (roughly speaking) convert SDTV (4:3) aspect ratios. Manufacturers build three options into many HDTV receivers:1. Zoom 2.Stretch 3. Combined zoom/stretch.

This is correct, some receivers can do this.
And even cheap ones would easily scale down HD to SD.
Though, output would then be either HDMI or analogue (if allowed by HDCP), never an MPEG2 stream.

Reason is, they have dedicated hardware for such purposes.
Most computer tuners don't.

And in fact, substituting a clever DSP chip by software solutions can result in a giant load for your CPU(s). A high tech graphics chipset might reduce that load, to decode h.264 for your computer screen or TV out, but AFAIK this would normally not help on recoding to MPEG2 on-the-fly there after.
Extra computer hardware for recoding would be required, but this might bring some severe legal trouble for manufacturers, once this accepted input from decoded h.264 resp. HDMI origin, that's supposed to often be protected by HDCP.

Of course the play out centres of TV stations and cable providers are able to transcode h.264 to MPEG2 (or vice versa) in close real time (e.g. with a delay of around four to five seconds), but that equipment won't be based on a usual personal computer...