You can :-
1) File a civil suit under Indian Contract Act on your old company.
2) File a case under 420 IPC, as this is cheating. As your company made a promise it never intended to keep.
3) Go to labour Court under Industrial Employment Standing Order
4) Go to labour Department Under Shops and Establishment Act
Supplementary salary is the additional remuneration paid to the employees in the form of allowances or otherwise over and above the basic wages. The speciality of this supplementary salary is that whatever paid under these heads no consequential effect on certain statutory indirect monetary commitments like gratuity and not exempt from Income Tax.
The course of remedy in the described situation is determined by the status of your employment i.e whether you are a "workman" as per section 2(s) of the Industrial Disputes Act,1947 at the time of merger and after as well or you were initially employed as a workman but subsequently elevated to that of supervisory/ administrative/ managerial cadre after the merger.
If you are a workman before and after the merger, the new management can not modify the above service condition in a less favourable way as per the condition stipulated u/s 25-FF(b) of the ID Act,1947.
In either case you are eligible for superannuation benefits at the rate prevailing before the merger.