If you can't change your opponent's monster to Defense Position, you can't apply the rest of the effect.
Your opponent chooses which effect to apply after their monster is changed to Defense Position.
Mentions in Other Rulings
The two effects are supposed to be read from your opponent's perspective - that is, your opponent gains Life Points, or you take damage. This is clear in the OCG text , and the OCG text of "  Dark Coffin" agrees.
1.0 1.1 1.2 1.3 1.4
Konami Gameplay FAQ: Absolute Powerforce - Card Rulings (Version 1.0)
Konami Gameplay FAQ: Duelist Revolution Sneak Peek -- Card Rulings (Version 1.0) ↑
3.0 3.1 3.2 3.3
Konami FAQ: Trap Card > Changing Destiny ↑
Konami FAQ: Trap Card > Dark Coffin
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