Hello, gaurangarya, since the rule book mentions the 20% penalty, etc., I am not sure if you have a choice. I am assuming that you accepted these terms when joining this company [basically Offer Acceptance]. Usually, for many, aspects of an Appointment Letter are not given much thought until things reach a critical stage, like it did for you. Maybe if you had perused the rule book BEFORE you resigned, you might have factored this aspect into your plans.
Policy Changes and Notice Period
Regarding your statement, "At the time of joining four years back, the contract had a 1-month notice period," a company can change the policies anytime. However, they must inform the employees about the changes through any commonly used means. Even if the company didn't, it could be tough to prove.
Frankly, I guess you don't have any option other than going by the rules now. Next time, please go through any offer with a fine-tooth comb before signing on the dotted line.
All the best.
Regards,
TS