Actually, an old film can easily be re-mastered in 1080i/p video and produce beautiful video that will surpass any DVD-quality transfer. However, except for companies like Criterion, don't expect to see this happening. Also keep in mind that you may well not SEE the differences on any screen smaller than about 60". Also keep in mind that most DVD's tend to run at about 1/2 the potential bitrate most of the time. So there's a lot of room for improved video even on a DVD if you remove the extra audio streams and extras. It's always about bitrate, whether it's a DVD or BD disc.

More often than not, I expect to see HD discs with DVD-quality video on them. I don't expect to see any studios doing expensive new transfers to get true HD quality, except for promotions and special editions.