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.
server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
import org.elasticsearch.cluster.ClusterStateTaskListener;
import org.elasticsearch.cluster.SimpleBatchedExecutor;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Priority;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.UUIDs;
import org.elasticsearch.common.ValidationException;
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.inject.Inject;
import org.elasticsearch.common.logging.HeaderWarning;
import org.elasticsearch.common.regex.Regex;
import org.elasticsearch.common.settings.IndexScopedSettings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.set.Sets;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.Index;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexService;
import org.elasticsearch.index.IndexSettingProvider;
import org.elasticsearch.index.IndexSettingProviders;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.index.mapper.MapperParsingException;
import org.elasticsearch.index.mapper.MapperService;
import org.elasticsearch.index.mapper.MapperService.MergeReason;
import org.elasticsearch.index.mapper.RoutingFieldMapper;
import org.elasticsearch.indices.IndexTemplateMissingException;
import org.elasticsearch.indices.IndicesService;
import org.elasticsearch.indices.InvalidIndexTemplateException;