Why at all you want to do this? Your organization has to be on the equally liable.
You cannot change the terms of service on the existing employees unilaterally. If you do so it will be change of their service conditions (to which company has already contracted with the employee) it will be a violation of the standing order and law in general. You have to have the consent of all such employees before doing so. Do not take innocent employees to ransom by unilateral pronouncements. It is bad HRM practice. Such a wild move can even bounce back to the disadvantage of the organization.
Mere putting the same on the notice board or bringing into the notice of the employees will not meet the end of law. If you have Union, then it is more appropriate that you get the consent of the union first.
Only thereafter, you can modify the Standing Orders and the get the same vetted from the law enforcement agencies, before they put to effectual practice.
Otherwise, it will be an Unfair Trade Practice and put your organization into trouble later or sooner. Moreover, it may hit the branding of the organization too, being unfavorable service conditions.
There are other ways to retain the employees rather then to coerce them to serve longer notice period or part with the substantial money. Remember that an unwilling employee is a liability on the company. Once an employees makes a mind to quit, he will do so, despite your embargo.