SD/HD and the Analog/digitial conversion have absolutely nothing to do with each other, so it doesnt matter if they have HD versions up or not by the time the conversion happens
Look at the issue in marketing terms rather than the mere nuts and bolts technological angle.
Any network still broadcasting only in SD at this point isn't going to look very good from the stand point of professionalism. They're either cheap or just don't have their act together. The Weather Channel is available in HD. How does that make other channels with "entertainment" on them look if they're still only in SD?
I
try to watch certain TV or cable broadcasts in SD at times -when there is no HD alternative. Unfortunately, I don't have much tolerance for it. I admit I am now very spoiled by high definition quality. If the show isn't in HD chances are I am not going to watch it. As "shallow" as that may seem, there's a strong chance a lot of other people feel the same way.
Once that change to all digital broadcasts takes place it's going to put far more public attention on the differences between SD and HD. The ones that want to stick with SD only for quite some time to come will have to make due with fewer and fewer viewers.