Teams swap models like they are changing a database driver. They are not.
A model switch changes the product. It changes how often the system refuses. It changes how verbose the answer is. It changes whether the output is cautious, eager, terse, or strangely confident. It changes latency and cost. It changes the class of mistakes users see.
If the user can notice, it is a product change.
Treat it like one
Before switching models, write down:
- The expected win. Lower cost, better accuracy, lower latency, wider context.
- The likely regression. Every model has one.
- The comparison set. Use real recent inputs, not the five examples in the launch doc.
- The rollback trigger. Decide what failure rate sends you back.
The eval does not need to be grand. It needs to be specific enough to catch the behavior your users rely on.
Model vendors sell interchangeability because it helps adoption. Product teams should be more suspicious. The model is part of the interface now. Version it like you mean it.