Heering Cherry liqueur is made to an 1818 recipe from Danish Stevns cherries, harvested in August. The fruit is crushed with the stones, giving Heering...
Luxardo Maraschino has a very different flavor from Cherry Heering. I'd first reduce the amount of Heering, to keep it from overwhelming while preserving the original flavor of the cocktail. Swapping Luxardo for Heering would yield a different drink altogether.