I'm just now starting to shoot in high definition and am fairly new to using after effects too. I wanted to know if there is an easy way to export a comp to the standard NTSC 720X486 when I'm done putting the effects in an HD shot, or if I'd have to redo all the effects in a brand new comp?
Nest the original HD main comp it in a NTSC 16:9 comp, run Layer --> Fit to Comp. Render. Done.