// {X}: Creatures you control become X/X and gain all creature types until end of turn.
DynamicValue variableMana = new ManacostVariableValue();
Effect effect = new SetPowerToughnessAllEffect(variableMana, variableMana, Duration.EndOfTurn, filter, true);
effect.setText("Creatures you control become X/X");
Ability ability = new SimpleActivatedAbility(Zone.BATTLEFIELD, effect, new VariableManaCost());
effect = new GainAbilityAllEffect(ChangelingAbility.getInstance(), Duration.EndOfTurn, filter);
effect.setText("and gain all creature types until end of turn");
ability.addEffect(effect);
this.addAbility(ability);
}