If {Scion of the ur dragon} is your commander, you attack, no blocks, turn into X dragon per ability, will the damage assigned still be commander damage since Scion is now a copy of X dragon until end of turn?
903.3. (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=903.3.): Each deck has a legendary creature card designated as its commander. This designation is not a characteristic of the object represented by the card; rather, it is an attribute of the card itself. The card retains this designation even when it changes zones.
Example: A commander that's been turned face down (due to Ixidron's effect, for example) is still a commander. A commander that's copying another card (due to Cytoshape's effect, for example) is still a commander. A permanent that's copying a commander (such as a Body Double, for example, copying a commander in a player's graveyard) is not a commander.
So yes it is commander damage.
(For the lazy people wanting a fast answer)