SigAtk1 Change?

I just read about how 70 damage attack can be 2 TCD...So can I go ahead and change it?

Also, I was hoping if I can change my SigAtk from 70 Damage into 60 damage/Raises Evasion for this turn Or do I need to buy something to change it...
Approved at 60 fire, Increases Evasion this turn, 2CD.