For Anime, it depends on the show.
On some series, like Ghost in the Shell: SAC there are already HD source materials as it was originally aired on Japanese TV in HD. There are a few other shows like this, such as Godannar.
On older series, I think it depends on the source material. If it's a movie, it is possible they could do an HD version, depending on the film used and all that. As far as TV Anime, I don't believe there is a way to create an HD source from a non HD master. Not sure exactly how HD works.
Even if they could create HD sources from standard, I don't think many shows would get that treatment. I mean, they'd have to go through a lot of bullshit for what I think would likely be little gain.
ADV canceled it, but never really said why. My guess is that it probably wasn't that much of a difference over the regular versions, or not enough to justify doing HD versions. A lot of people complained about the HD version they were using, that being Windows Media, as it's full of copy protection BS, along with region protection crap, meaning someone outside the US wanting it, wouldn't be able to play it.