How to properly serialize data and keep old API contract?

Question

I changed external API url, from old API url response had current format:

[
  {
    "id": 2222
  },
  {
    "id": 3333
  },
  {
    "id": 4444
  },
  {
    "id": 5555
  }
]

after URL change response looks like this. More fields and id type is String.

{
  "ref": {
    "id": "1111"
},
  "compset": [
    {
        "id": "2222"
    },
    {
        "id": "3333"
    },
    {
        "id": "4444"
    },
    {
        "id": "5555"
    }
],
  "isCompliant": false,
  "updateTimestamp": null,
  "remainingMilliseconds": 0,
  "totalLockoutDays": 15
}

All I need is a just compset list. Other fields can be omitted.

Response classes to serialize this json implemented in two classes.

class 1:

public class Compset {
    private List<Competitor> competitors;
}

class 2:

public class Competitor {
    @JsonProperty("id")
    private Integer id; // 
}

This code defines how to get data from old external api with feign:

public interface PropertyCompetitorConnector {
    @Headers({"Accept: application/json"})
    @RequestLine("GET /api/v1/compset?id={id}")
    Observable<List<Competitor>> getPropertyCompetitors(@Param("id") Integer id);
}

This code makes request saves data in cache and returns it:

 private Observable<List<Competitor>> callApi(final Integer id) {

    final PropertyCompetitorConnector connector =
            feignConnectorBuilder.buildJacksonConnector(someConnector.class);

    Observable<List<Competitor>> responseObservable = connector.getPropertyCompetitors( id)
            .doOnNext(response -> {
                Compset compset = new Compset();
                if (response != null) {
                    comspet.setCompetitors(response);
                    cacheHandler.put(someCacheName, String.valueOf(id), Compset);
                }
            })
            .doOnError(t -> {
                //error handling
            });

    return responseObservable;
}

How can I do refactor so my service works as same and returns data as same as before even the received json is different?