Currently it is possible to migrate an existing data stream to a time series data stream.
This is done by updating the index template that created the data stream. 
Then after a rollover the data stream is a time series data stream. 

Via a similar way it should be possible to change time series data stream back 
to a regular data stream. Currently when changing the `index.mode` setting
on an index template from `time_series` to `standard` (or removing the setting)
results in a validation error.
modules/data-streams/src/test/java/org/elasticsearch/datastreams/MetadataIndexTemplateServiceTests.java
createTestShardLimitService(randomIntBetween(1, 1000)),
            new Environment(builder().put(Environment.PATH_HOME_SETTING.getKey(), createTempDir().toString()).build(), null),
            IndexScopedSettings.DEFAULT_SCOPED_SETTINGS,
            null,
            xContentRegistry(),
            EmptySystemIndices.INSTANCE,
            true,
            indexSettingProviders
        );
        return new MetadataIndexTemplateService(
            clusterService,
            createIndexService,
            indicesService,
            new IndexScopedSettings(Settings.EMPTY, IndexScopedSettings.BUILT_IN_INDEX_SETTINGS),
            xContentRegistry(),
            EmptySystemIndices.INSTANCE,
            indexSettingProviders
        );
    }
    public static ShardLimitValidator createTestShardLimitService(int maxShardsPerNode) {
        
        ClusterService clusterService = mock(ClusterService.class);
        Settings limitOnlySettings = Settings.builder().put(SETTING_CLUSTER_MAX_SHARDS_PER_NODE.getKey(), maxShardsPerNode).build();
        when(clusterService.getClusterSettings()).thenReturn(
            new ClusterSettings(limitOnlySettings, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS)
        );
        return new ShardLimitValidator(limitOnlySettings, clusterService);
    }
}