I think that the answer might depend on what shape the existing coolant is in. I changed mine last fall, even though the stuff in the radiator looked good as new. However, it was over 3 years old and time to go. So, I only removed the drain plug on the bottom of the engine case. Tilted and rocked the bike a bit to try and get most of it out. I figured that the little that was left would do much harm and it has not.
Now, if the existing coolant were all brown and nasty, then I would have opened all the drain plugs and flushed it well to make sure that I got it all out.
You decide what's need for your situation.
Also, bleeding does not require removal of the gas tank. The bleeder is easily reached from the front. Tip: attach a clear bleeding hose to the bleeder, loop it straight up and then down into a bucket. It will prevent coolant from running down the engine/bike and give a clear indication when then system is free of air. Just look for the coolant to extend above the bleeder when open. Reach in with rag and pinch it when you pull it off to catch the last bit of coolant.
Chris Glennon - Portland, OR