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
this(in.readString());
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            out.writeString(name);
        }
        @Override
        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
            builder.startObject();
            builder.field(NAME_FIELD.getPreferredName(), name);
            builder.endObject();
            return builder;
        }
        public String getName() {
            return name;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            TimestampField that = (TimestampField) o;
            return name.equals(that.name);
        }
        @Override
        public int hashCode() {
            return Objects.hash(name);
        }
    }
}