import os, time
test = []
os.chdir('C:\\Users\\Michael\\mIRC')
with open('mirc.ini', 'r') as f:
    for line in f:
        if line.startswith('lastrun'):
            test.append('lastrun=\r\n')
        else:
            test.append(line)
print('File modified')
with open('mirc-new.ini', 'w') as f:
    for line in test:
        f.write(line)
print('file written')
os.rename('mirc.ini', 'mirc-{}.ini'.format(int(time.time())))
print('old file preserved and renamed')
os.rename('mirc-new.ini', 'mirc.ini')
print('new file in place')