You have to file petition in central administrative Tribunal for pay protection 

if your representation was rejected in 2016 you should have immediately file petition before CAT 

Pay protection is a benefit given to candidates who join Central Government posts on direct recruitment basis from other sectors such as PSUs, State Governments, etc. Pay protection means that their initial pay in the Central Government post is fixed at a stage that protects their pay and allowances drawn in their previous sector. However, there are certain conditions and criteria for availing this benefit, such as:

• The candidate must have completed the probation period successfully in the previous sector and obtained a regular or confirmed status.
• The candidate must have applied for pay protection within one month from the date of joining the Central Government post.
• The candidate must have submitted the necessary documents such as NOC, pay certificate, service certificate, etc. from the previous sector.
• The candidate must have fulfilled the minimum experience requirement prescribed in the recruitment rules for the Central Government post.
• The candidate must have joined the Central Government post through a proper selection process such as interview or competitive exam or both.

In your case, it seems that you have applied for pay protection in CPSU after joining from a State PSU in an experienced post. However, your application was rejected by your present company on the basis of lower CTC in your previous company and the rule of their major stake holder CPSU company. You have also submitted various documents and representations to review your application, but your present company has not responded positively. You have also found out that your present company has provided pay protection to other employees who joined from various CPSU and Central Govt Sector with lower CTC than yours. You have also discovered that your present company has framed a new rule for pay protection that segregates the industries in two categories, one for all CPSU and another for Central Govt/State Govt/State PSU etc. You have also received an RTI reply from your present company that provides this rule reference to whom they provided pay protection 10 years before.

Based on these facts, you may have a valid ground to file petitions in court/outside platform for pay protection. You can take the following steps to do so:

• 
  Collect evidence: You should collect all the relevant evidence to support your claim for pay protection, such as:

• Your appointment letter and joining report in CPSU
• Your NOC and pay certificate from State PSU
• Your service certificate and experience certificate from State PSU
• Your pay slips and CTC details from State PSU and CPSU
• Your application and representations for pay protection in CPSU
• The rejection letter and RTI reply from CPSU
• The recruitment rules and eligibility criteria for your post in CPSU
• The pay protection rules and guidelines issued by DPE or DOPT or any other authority
• The details of other employees who got pay protection in CPSU with lower CTC than yours

• 
  Choose a forum: You should choose a suitable forum to file your petitions, depending on the nature and value of your claim. Some of the possible forums are:

• 
  Central Administrative Tribunal (CAT): This is a quasi-judicial body that deals with service disputes of Central Government employees. You can file an original application (OA) in CAT seeking quashing of the rejection letter, direction for pay protection, declaration of your rights, and compensation for loss and harassment. You can also file an interim application (IA) seeking stay of any adverse action against you by CPSU pending disposal of OA. You can file an OA in CAT within six months from the date of rejection letter or any other cause of action.
• 
  High Court: This is a constitutional court that has jurisdiction over all matters arising within its territorial limits. You can file a writ petition in High Court under Article 226 of the Constitution of India seeking writs of mandamus, certiorari, prohibition, quo warranto, or any other appropriate writs or orders against CPSU or any other authority for violation of your fundamental rights or legal rights. You can also file an interim application (IA) seeking stay of any adverse action against you by CPSU pending disposal of writ petition. You can file a writ petition in High Court within three years from the date of rejection letter or any other cause of action.

If you are aggrieved by the decision at that time, then you should have taken action on it then itself instead of waiting for the developments and try to fill up the lacuna now. However you can approach CAT  with your grievances with an OA  and seek relief and remedy for the reasons you rely upon.
In common parlance, the employee shifting to Central government job from State or Central PSU are eligible for pay protection whereas it is not vice versa.

There is no provision in CPSU to protect the pay of the employees joined from state government or state PSUs. 

Dear Sir,

You are suggested to make one strong representationa then apporach High Court under writ juristiction. 

Greetings of the Day,

I have read your issue but find that there are some facts which need to be clarified. You can approach on either nine two one two one two four five eight five or seven zero four two eight three three six eight six for personal approach.

We Hope that a personal consultation would be fruitful to the issue and it would be better for you to talk to someone directly.

Best Regards

SPS Law Chambers
New Delhi.