Doran, the Siege Tower

• WWizards of the Coast2007-10-01
  Official Ruling
  Doran's ability doesn't actually change creatures' power; it changes only the value of the combat damage they assign. All other rules and effects that check power or toughness use the real values.
• WWizards of the Coast2007-10-01
  Official Ruling
  Doran's ability means, for example, that a 2/3 creature will assign 3 damage in combat instead of 2.
• WWizards of the Coast2007-10-01
  Official Ruling
  This effect is mandatory and affects all creatures.

Rulings via Scryfall