Try this if you have your original SD composition:
1) Create a new HDTV 1920×1080 composition.
2) Drop your SD composition into the HD composition.
3) Enable ‘Collapse Transformations’ on the SD comp layer.
4) Use Layer > Transform > Fit To Comp Width (or Height, whichever doesn’t leave black area around your comp) on the SD comp layer.
If your images in the SD comp are high resolution and scaled down below 100% then they will maintain their original high resolution when scaled back up to HD. Meaning that if your images are scaled down to something like 45% in your SD comp, when you scale the SD comp up by about 225% (after enabling ‘Collapse Transformations’) your images will effectively be scaled to 100% and you will not lose any resolution. Vector images will scale flawlessly as well if they are set to continuously rasterize.
This all depends on the original resolution of your images however.
Darby Edelen
Designer
Left Coast Digital
Santa Cruz, CA