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Not to be confused with the swagger framework versioning, the swagger specification itself is versioned for compatibility. You can see information about previous versions in the wiki.
Philosophically, any changes to the spec are absorbed into the tools for one-version backwards compatibility. So the swagger-ui will work with both swagger-spec 1.1 and 1.2. It is not tested against older versions (prior to 1.1). Changing a specification typically causes some heartache and pain, so the changes introduced are usually not just for fun--they have a history of issues or confusion behind them, which mandates the change.
You can use the swagger-codegen to migrate your swagger 1.1 spec! Please see HERE for details.
That said, here's a transition guide for the 1.2 specification.