Used this script to unpublish all jekyll posts before a particular year

unpublish.py

import os
PATTERN = "author: leonson"
YEAR = 2012

def unpublish(filepath, pattern):
	file_handle = open(filepath, "r")
	contents = file_handle.readlines()
	file_handle.close()
	index = 0

	for i in range(len(contents)):
		if pattern in contents[i]:
			index = i

	contents.insert(index+1,"published: false\n")

	f_write = open(filepath, "w")
	contents = "".join(contents)
	f_write.write(contents)
	f_write.close()

def process_folder(path):
	file_list = os.listdir(path)
	for entry in file_list:
		year = int(entry.split('-')[0])
		if(year < YEAR):
			file_path = "/".join([path, entry])
			unpublish(file_path, PATTERN)