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Effects that set a creature's power, toughness, or creature types will be applied in timestamp order. The timestamp of Futurist Operative's first ability is when it entered the battlefield, not when it became tapped. Notably, this means the ability will not overwrite any effects that set its power and toughness to specific numbers after it entered the battlefield. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

Untapping Futurist Operative once it's been declared as an attacker won't remove it from combat.