Can you change the bulbs out of the fog lights on the 98-01 imprezas? Or do you have to buy a whole new foglight assembly?

Yes, and no. To access the bulb you remove the foglight assembly from the bumper (3 10mm bolts), there is a cap on the back that twists off and the bulb is under that.
The bulbs are H3. Replace them both together since once one blows the other will soon follow. Also this will keep the light output the same on both lights.

cool, thanks.