These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
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).
Divine Wrath: You cannot activate "Divine Wrath" when..."Spear Dragon" changes to Defense Position because these are considered Continuous Effects.
Dragon's Rage: If you attack with "Spear Dragon"...the difference between your monster's ATK and the attacked monster's DEF will only be inflicted as Battle Damage once. The effect of "Dragon's Rage" will not inflict the damage again.
Kishido Spirit: When "Spear Dragon" attacks a Defense Position monster with equal ATK to "Spear Dragon", perform damage calculation normally even though the Defense Position monster is not destroyed.
Magic Cylinder: If the attack of..."Spear Dragon" is negated by the effect of "Magic Cylinder", they will not be changed to Defense Position, since Damage Calculation never occurred.
Skill Drain: ..."Spear Dragon" do not change their battle position after attacking while "Skill Drain" is active...
Spirit Reaper: While in Defense Position, "Spirit Reaper" will stop all Battle Damage to your Life Points from most monsters, but monsters like "Spear Dragon" and those equipped with "Fairy Meteor Crush" can still inflict Battle Damage to your Life Points with their effects.
Spirit Reaper: Examples of cards that will not destroy "Spirit Reaper" because they do not target: ..."Spear Dragon".
Tragedy: "Spear Dragon" changes its battle position at the end of the Damage Step, so you cannot activate "Tragedy" at that time...