Two options, IMHO. If you demand suspension for the off-road riding, leave it as is. I did this for ~4,000 commuting miles and it worked just fine.

Now that I have upgraded my MTB (to a Giant Trance X2), I changed my Kona Caldera (a HT MTB) that I had been commuting on to a utility hauler. As I no longer ride it off-road and I don't ever want/need front suspension, I swapped the fork out for the Surly 1x1. This set-up is actually better for utility hauling as it is more rigid and can handle more of the added stress applied to the fork via the disc brakes.

To piggyback on what fietsbob said, I don't think it would take me more than 5 minutes to switch a threadless fork out, though I've never timed myself. It's definitely not a big job and easy to switch back and forth, if you don't mind the extra hassle. It's 5 - 5mm allen bolts to loosen/undo and then re-tighten. Of course, if you'll potentially have to realign the front brake (again, a very simple, quick task).