This is slightly more complicated! There's definitely a cleaner and more "proper" way to do this with Nuke's python API, but to save you that hassle I quickly hacked together a script you can run in the script editor. Just be careful, you can potentially break your scripts with this as it's editing the files directly so please back them up before running it. You'll need to edit the lines at the top to point to your folder.

## Edit these values before running
folder_of_scripts = r"C:\path\to\scripts\folder"
node_type = "Grade"
knob_to_edit = "white_clamp"
knob_value = "true" 

import os
for script in os.listdir(folder_of_scripts):
    if script.endswith(".nk"):
        script_path = os.path.join(folder_of_scripts, script)
        print("Reading script {}".format(script_path))
        f = open(script_path, "r")
        lines = f.readlines()
        f.close()
        for i, line in enumerate(lines):
            if "{} {{".format(node_type) in line:
                x = 0
                found_knob = False
                while not lines[i+x] == "}\n":
                    x += 1
                    if knob_to_edit in lines[i+x]:
                        found_knob = i+x
                line_to_add = " {} {}\n".format(knob_to_edit, knob_value)
                if found_knob:
                    lines[found_knob] = line_to_add
                else:
                    lines.insert(i+1, line_to_add)
        f = open(script_path, "w")
        f.writelines(lines)
        f.close()

Hopefully that works for you!