i have rendered an HD project in after effects which was 1027x728. as we are sendeing it to be broadcasted on tv, they have requested it to be 720x576.
i made a composition in ae, sized 720x576 , the same fps (25), and i put the rendered project in it and scaled it, and then i used letterboxer to fill the blank upper and lower areas.
then i render this down'rez composition.
the problem is:
the is quality loss, specially the titles and texts seem blured and noisy.
You could render from your original comp and tell AE to resize the image to 720x576 with a widescreen pixel aspect ratio. There is no need to letterbox unless you know all screens this will be send to are 4:3 (most likely the vast majority will be 16:9 in which case your piece will look right)
Quick tip, when rendering from AE, no need to render twice if you need various formats - hit the "+" next to your output module and add another output size. AE will render once and produce two files (my mouth was open for a while when I learned about this feature...).