Jack up the bike, remove both fork upper caps, pull spacers and springs, then lower the bike to compress the forks all the way. Measure the amount of old oil in the forks. Draw out as much oil as possible using a hose and one of those large syringes. This will get most of the oil out. Refill to the level you previously had when starting, install springs, spacers and caps.
Not perfect (doesn't get all the oil) but relatively easy.
