Migration Plan for Model Switch

Plan a migration from [CURRENT MODEL] to [TARGET MODEL] for a production workload.

Cover:

1. BEHAVIOURAL DELTAS to expect: tokenizer differences and their effect on cost, system prompt interpretation, refusal boundaries, structured output strictness, tool-calling format, streaming behaviour.
2. WHAT WILL SILENTLY BREAK: the changes that pass tests and fail in production. Be specific.
3. TOKEN COUNT IMPACT: different tokenizers mean the same text costs different amounts. Estimate the direction and rough size.
4. SHADOW TEST PLAN: how to run both models on real traffic and compare, including what to compare on and the sample size needed.
5. ROLLOUT: percentage stages, what metric gates each stage, and the rollback trigger.
6. WHAT TO DELETE AFTERWARDS: the workarounds for the old model's quirks that should not survive the migration.

Workload:
[DESCRIBE]