YouTube uses a blend conversion, ie. it will mix the various frames together at an appropriate ratio. So while you in principle get all of the images jammed in there, the result is quite blurry (has a double exposure look to it). Whether it's better than just decimating to 25fps is a matter of taste, but I'd say it isn't (30 fps is only a 20% improvement over 25, after all). Rather just have a clear 25fps YouTube video and a true 50fps version for download somewhere else.