The quality of these rubber tubes are not pliable enough to stand the test of time, even mine on a one year old PCX has hardened to the point it most likely won't be reusable. Squeeeeze the tabs on the retaining clip and slide the little compression coil down to the lower end of the rubber tube. At this point all the muscle you have may not allow the rubber to expand enough to slide off the drain port. These are plastic so you could snap the whole thing off the air box trying with tools
At this point I would razor off the old tube and consider it a loss seeking to replace it with a fresh clear rubber tube. The lack of elasticity of the clear rubber is where I underestimated your boggle now... I apologize after attempting to remove mine. You are correct to be frustrated, perhaps some heat would actually bring softening to the original tube but then we are back to setting the bike on fire LOL
but, Why bother? We're getting them off! 
