It's easier to flush and re-fill with the top off. I got away without a new gasket as the old one didn't rip. Don't over fill them, the oil only needs to cover the mechanism inside so about up to the level of the gasket face.
To flush them put the bottom plug in without the valve, clamp the damper in a vice by the bolt lugs, fill with fresh oil and work the lever up and down. Do this a couple of times to get all the crud out then replace the valve, fill with oil and again work the lever up and down. You will feel the resistance build up to the point where it will be difficult to move the lever so it needs to be secure in the vice (good idea to bolt the damper to a plank of wood). Top the oil level up and job done.