Tengu format ruling
Spear Dragon

Card effect (current official text)During battle between this attacking card and a Defense Position monster whose DEF is lower than the ATK of this card, inflict the difference as Battle Damage to your opponent. If this card attacks, it is changed to Defense Position at the end of the Damage Step.
Rulings
- If Spear Dragon attacks, even if the damage is reduced to 0 by the effect of Waboku or Kuriboh, it is still changed to Defense Position.
- Spear Dragon will change to Defense Position after it attacks, regardless of the battle position of the opponent’s monster.
- The effect of Spear Dragon does not prevent you from manually changing its battle position later on. Of course, you cannot change it in the same turn after you attacked with Spear Dragon. But during your next turn you can change it back to Attack Position (and attack again the same turn, if you choose).
- If you attack your opponent’s Defense Position monster with 0 DEF with your Spear Dragon, and have Dragon’s Rage active on the field, the opponent only takes 1900 points of damage. The effects are not cumulative.
- The effect of Lord of D. will not prevent the effect of Spear Dragon from changing it to Defense Position after it attacks.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Tengu forum — sourced change requests get reviewed.