The usual way - by reloading the preset from library having the On Duplicate Material - Replace from Library option marked isn't recommendable in this case, as it would bring also the preset's settings.
I would suggest you to create a new Forest object - just to import such material version into your scene (in case of Lawns presets the "_fp_grass" as on attached screen-shot) and that one copy & paste to existing Forest objects (under Geometry - Properties - Material).
Let me add, that all bundled material libraries are stored by default at:
C:\Program Files (x86)\Itoo Software\Forest Pack Pro\matlib
in several files corresponding to used renderer (3d_vray.mat, 3d_thea.mat, 3d_standard.mat, 3d_octane.mat, 3d_mental.mat, 3d_corona.mat) and it's possible to assign them directly, but to avoid any naming issues, I would suggest you the "new Forest object" work-around explained above. Hope that helps.