Dear Ms. Sashmita,
I had understood from the information that I had acquired through various resources, including my experience, that "SIMPLICITY" is missing in those employees holding senior-most positions with decades of experience.
1) I believe either POWER, MONEY, KNOWLEDGE, SKILLS/TALENT, or BACKGROUND SUPPORT is making senior employees develop and promote EGO. These employees know how to play the SAFE GAME and RETAIN (they know how to convince their boss, butter them up, and a lot more) their POST until they retire. However, whoever challenges or takes initiative to bring changes in them will cost him/her the job.
2) The greatness of an individual lies in his simplicity. Those employees who have embraced SIMPLICITY are the BEST LEADERS, blessed with multiple skills, and loved by all. Also, they maintain very LOW SELF-ESTEEM.
3) When your HOD'S are supported by DIRECTORS & CEO, you can't do anything. I suggest the practical solution "CHANGE SHOULD COME FROM TOP MANAGEMENT".
4) If you handle them diplomatically, you will become one among them as they want you to be like "what they are".
5) If you deal sternly, then it may cost you your job as they are in the good books of DIRECTORS & CEO.
If you really want to change them gradually, which is a time-consuming process, then I suggest you adopt a process where you need to let them know and differentiate between GOOD and BAD, ANGEL & DEVIL. They have to understand "what they are" and "what they are supposed to be". You need extensive knowledge, maturity, a lot of patience, positive mental/physical attitude (positive thinking and a lot more to deal with such people... Any time you may lose the whole game, but never let this moment happen...
It is not you who is going to change your HOD, but it is they themselves who are going to change themselves... you are just going to help them embrace positive changes. GOOD LUCK...
With profound regards