I always prefered Luke using a white lightsaber in the training scene on the Millenium Falcon, like it has been in many of the home releases.
I imagine it to be a training mode with a less intense nonlethal blade for the lightsaber so he doesn't kill himself if he swings it around and cuts himself by mistake, though its strong enough to reflect those very weak blaster bolts being shot by the droid. Using his father's lightsaber in full blue killing mode when he's at the very beginning of his training seems very foolish to me.
It also seems kinda disrespectful to his father's lightsaber and doesn't fit in with the beliefs of the Jedi, the Jedi like Samurai seem to hold their lightsabers in high regard, they even have to build their own themselves as part of their trials, like learning samurai would use wooden sticks to train, i'd imagine jedi apprentices would use weaker nondeadly lightsabers to train rather than fully fledged lightsabers so they could spar without killing each other.
Good change by GL as far as i'm concerned. Makes much more sense and fits better with the mythology IMHO.