Discremation in service condition

Presently, I am working for a PSU for the last 11 years. I have joined 2004 as Asst. Manager (Finance). The facts of services which raised certain concerns are as under; a) In June 2008, I have been promoted to next grade Dy. Manager in same PSU, through open advertisement (i.e. written exam and interview). In the said year (i.e. from April-2008), I was also due for promotion through Department Promotion Policy (DPC). However, as promoted through the open advertisement, I was not allowed to appear for DPC. Other candidates of the said Corporation, who were failed against the open advertisement and Joined in 2006 (i.e. 2 years junior to me) were promoted through DPC (just 10 days after my joining through the open advertisement) without completing by them the eligible years of service (i.e. minimum 3 years service condition) and they have offered the promotion from April-2008. b) As, I have joined through open advertisement in May – 2008, I was deprived from maximum Individual Production Incentive (i.e. 12% rate) instead of allowed minimum rate (i.e. 3%) implemented in year 2009 as only those employees, who have at least promoted to next grade through DPC were made eligible to draw maximum individual production incentive, c) In July 2011, I have been promoted to next grade Manager in same PSU through DPC, d) Promotion to next cadre (i.e. manager to senior manager), the minimum eligible period is of 4 years. Previously, the Performance Report (CR) period was from July to June. However, keeping in view to grant promotion on time (i.e. in July), this year CR period of made of 9 months (i.e. From July – 2014 to March – 2015) as special measure. Accordingly, I was due for promotion to next grade as of March-2015. In December – 2015 (Board meeting), the PSU have changed the promotion HQI and made the minimum eligible period of 5 years in place of 4 years. I have again been debarred from appearing for promotion. - Whether the said PSU can change my service condition (i.e. promotion policy) unilaterally. Promote few employees on four year basis, few employees on five years basis. It is a clear act of discrimination. - Grant few employee promotion to next grade without completing the minimum years of service, - Deprive me from PLIS just I have promoted thought open advertisement in spite of the fact that in same year I was also due for promotion through DPC On the basis of above facts, what the further course of actions are available to me.