“Ancient Forest” will not change a monster to face-up Attack Position if it was not on the field in Defense Position when “Ancient Forest” was activated.
Mentions in Other Rulings
Scrap Worm: If "Scrap Worm" attacks while "Ancient Forest" is active, you resolve the turn player’s effect first and then the opponent’s. If both are controlled by the same player, they can be resolved in any order.
The effect, "When this card is activated, if there are any Defense Position monsters on the field, then those monsters are changed to face-up Attack Position." of "Ancient Forest" is not applied at activation - it is applied when resolving the effect, after the opponent has had the chance to chain to the effect. If "Mystical Space Typhoon" is chained to the activation of "Ancient Forest", then "Ancient Forest" is not face-up on the field when its effect is resolving as Chain Link 1, so the monsters are not changed to face-up Attack Position.