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LucyBot api-spec-converter (online version, GitHub repo, Node.js module) can convert from OpenAPI 3.0 to 2.0.
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API Transformer (paid service) also claims to be able to convert OpenAPI 3.0 back to OpenAPI 2.0. It has a command-line version too.
Keep in mind that OAS3→OAS2 convertion is lossy in general, because OAS3 has features that did not exist in OAS2 (such as multiple servers, oneOf/anyOf, different schemas per media type, objects in query string parameters, cookie parameters, and others).