As per case facts, Petitioners were promoted as Assistant Engineers based on a 2006 notification that required "5 years service as JE/Civil." They received further promotions. In 2021, the Respondent-Nigam ... issued a retrospective "correction," claiming the original notification had an "inadvertent mistake" and intended to require "5 years service as such" (after degree acquisition). This led to a revised ranking list, making Petitioners liable for reversion. The question arose whether this retrospective "correction" could be applied to unsettle their accrued promotional rights and seniority. Finally, the High Court held that the original regulation was unambiguous, and the retrospective "correction" was a substantive amendment, not a mere clarification. Thus, it cannot be applied retrospectively to withdraw vested promotional rights and recast their seniority, and will operate only prospectively.