Downgrade ES from 2.3.1 to 2.0.0 with knapsack

[everkhoturov]

Please help in our problem. We are trying to perform a downgrade ES from 2.3.1 to 2.0.0 with elasticsearch-knapsack.
As result not all docs imported. ES log full similar messages like this:

[2016-05-27 21:09:31,940][DEBUG][action.admin.indices.mapping.put] [Master] failed to put mappings on indices [[gartender]], type [notifications]
java.lang.IllegalArgumentException: Mapper for [_id] conflicts with existing mapping in other types:
[mapper [_id] cannot be changed from type [_id] to [string]]
at org.elasticsearch.index.mapper.FieldTypeLookup.checkCompatibility(FieldTypeLookup.java:117)
at org.elasticsearch.index.mapper.MapperService.checkNewMappersCompatibility(MapperService.java:345)
at org.elasticsearch.index.mapper.DocumentMapper.addMappers(DocumentMapper.java:390)
at org.elasticsearch.index.mapper.DocumentMapper.merge(DocumentMapper.java:412)
at org.elasticsearch.index.mapper.MapperService.merge(MapperService.java:281)
at org.elasticsearch.index.mapper.MapperService.merge(MapperService.java:242)
at org.elasticsearch.cluster.metadata.MetaDataMappingService$2.execute(MetaDataMappingService.java:440)
at org.elasticsearch.cluster.service.InternalClusterService$UpdateTask.run(InternalClusterService.java:388)
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:225)
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:188)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

Please help solve this problem