Few channels I watch still use 4:3 AR sometimes. Problem is this:

I start the channel and AR is 4:3 (commercials), then movie follows in 16:9, but AR stays 4:3 (deformed). I have to switch channel or start the one again, AR changes to correct one.

Is it possible that ProgDVB could switch aspect ratio automatically?

Yes. ProgDVB support that. But not work with some video codecs. For example FFD not support that.

Ah, thanks! Custom EVR is the problem. With EVR it works

Arcsoft, Cyberlink, Elecard, Nvidia, MS DTV and even ffdshow (3488).