If the card has any mandatory additional costs, you must pay those if you want to cast the card
Just remember that whichever mode you choose to cast becomes the mana value of the card on the stack and on the battlefield, unlike other, similar built-in cost reduction mechanics like Suspend or Miracle, s o a commander like Yuriko, the Tigers Shadow very much likes Prototype cards, while Vial Smasher the Fierce does not
Red So we have Knowledge Pool, and we have Thousand-Year Storm, and well, Im sure they had a great time together, because now we have Arcane Bombardment
If you use a replacement effect which doesn't actually draw a card, you suffer no ill effects
Ultimately, I'm not super impressed by the Red cards in the set yet, because a lot of them seem to rely on equipment