with open('test1') as file_in:
    text = file_in.read()

text = text.replace("TEST_OLD", "TEST_NEW")

with open("test2", "w") as file_out:
    file_out.write(text)

oldfile = open("file", "r")
newfile = open("file_new", "w")
for line in oldfile:
    s = line.replace("TEST_OLD","TEST_NEW")
    newfile.write(s)