What TheGodBen said. They'll need to recut every episode. Which isn't as much work as redoing all of the vidual effects, but honestly, it wouldn't even be that hard.
And I'm gonna disagree that they won't make a return on their investment. Are you telling me that Trekkies are not going to buy up HD versions of TNG and VOY in droves?\
I see the reasons why it's harder and why they may not do it, but IMO, the pros far outweigh the cons, and if they look into it, I'm sure TPTB would agree that's it totally worth it.