In this study, we examine the transition from the U.S. Nuclear Regulatory Commission’s Systematic Assessment of Licensee Performance (SALP) to the Reactor Oversight Process (ROP), focusing on how the new inspection system was implemented and became operational in its early phase. Rather than treating regulatory reform solely as a change in institutional design, we conceptualize regulatory transition as a process in which design principles are translated into operational practice under existing institutional conditions. In the analysis, we adopt a C–X–Y framework in which path-dependent conditions (C) shape the quality of transition (X), which in turn affects regulatory outcomes (Y). Transition quality is examined as a process characteristic, focusing on planning efforts, adjustments between design and practice, and interaction among key stakeholders during implementation. Referring to historical documents and contemporaneous records, we show that although ROP was introduced under strong time pressure and political scrutiny, its operationalization was supported by adaptive adjustments and sustained interaction among regulators, industry, and third parties. These processes enabled ROP to emerge as a nationally consistent inspection system despite initial uncertainty and organizational friction. By emphasizing implementation rather than mature performance outcomes, we offer a process-based explanation of regulatory transition applicable to other policy domains.