Abstract
Organizations face a timing problem in post-quantum migration: deployed mechanisms may need to be retired, hybridized, or re-keyed before policy-defined security margins fall below asset-specific thresholds. This one-page submission outlines a compact, auditable pipeline that converts evolving assumptions and partial evidence into conservative start-by windows rather than single break dates. First, a scenario-conditioned security-trajectory layer represents margin erosion with a simple decay model and reports threshold-crossing times as sensitivity bands across plausible rates and policy thresholds. Second, a latent-regime layer uses a Hidden Markov formulation to represent partially observed risk states (healthy/watch/at-risk) and update posterior regime probabilities as evidence changes. Third, an option-style timing layer on a discrete lattice summarizes the diminishing value of delaying migration as thresholds approach, producing a ranked urgency view and feasible migration windows that explicitly account for lead time and governance buffers. Outputs are conditional on stated assumptions and are intended to be refreshed periodically as standards guidance and measurements evolve.