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/DataStream.java
&& indexMode == that.indexMode;
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, indices, generation, metadata, hidden, replicated, allowCustomRouting, indexMode);
    }
    public static final class TimestampField implements Writeable, ToXContentObject {
        public static final String FIXED_TIMESTAMP_FIELD = "@timestamp";
        static ParseField NAME_FIELD = new ParseField("name");
        @SuppressWarnings("unchecked")
        private static final ConstructingObjectParser<TimestampField, Void> PARSER = new ConstructingObjectParser<>(
            "timestamp_field",
            args -> {
                if (FIXED_TIMESTAMP_FIELD.equals(args[0]) == false) {
                    throw new IllegalArgumentException("unexpected timestamp field [" + args[0] + "]");
                }
                return TIMESTAMP_FIELD;
            }
        );
        static {
            PARSER.declareString(ConstructingObjectParser.constructorArg(), NAME_FIELD);
        }
        private final String name;
        public TimestampField(String name) {
            if (FIXED_TIMESTAMP_FIELD.equals(name) == false) {
                throw new IllegalArgumentException("unexpected timestamp field [" + name + "]");
            }
            this.name = name;
        }
        public TimestampField(StreamInput in) throws IOException {