Someone over on Reddit has noticed on the US PlayStation Store that there is an upgrade option available for Dishonored: Definitive Edition meaning that if you own the digital version of Dishonored on PS3 you can upgrade your game for half the cost of buying it new ($19.99) in order to play the Definitive Edition on PS4.
A great deal, especially for those who picked up Dishonored for PS3 when it was free with PS Plus back in April.