My thought is, someday, maybe, they'll devise a technique for up-scaling video to HD. Never know...
Any image can be up-scaled, including video image, if you watch a regular DVD on an HDTV either your DVD player or your TV will up-scale the image, otherwise the image will only take up a small part of the screen. The problem is that an up-scaled image can never reproduce the same level of quality as the film image because it doesn't contain the data and has to make approximations. The best up-scaler in the world will never be able to perfectly recreate the full-HD image.
A good example for this would be the bios for Archer and Hoshi in In a Mirror, Darkly
which were readable on HDTVs but were difficult to make out when displayed on an SDTV. No matter how well your up-scale that SD image the words will be blurry and difficult to make out while the proper HD image will be crisp and clear.