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
}
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        out.writeString(name);
        TIMESTAMP_FIELD.writeTo(out);
        out.writeList(indices);
        out.writeVLong(generation);
        out.writeGenericMap(metadata);
        out.writeBoolean(hidden);
        out.writeBoolean(replicated);
        out.writeBoolean(system);
        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
            out.writeBoolean(allowCustomRouting);
        }
        if (out.getVersion().onOrAfter(Version.V_8_1_0)) {
            out.writeOptionalEnum(indexMode);
        }
    }
    public static final ParseField NAME_FIELD = new ParseField("name");
    public static final ParseField TIMESTAMP_FIELD_FIELD = new ParseField("timestamp_field");
    public static final ParseField INDICES_FIELD = new ParseField("indices");
    public static final ParseField GENERATION_FIELD = new ParseField("generation");
    public static final ParseField METADATA_FIELD = new ParseField("_meta");
    public static final ParseField HIDDEN_FIELD = new ParseField("hidden");
    public static final ParseField REPLICATED_FIELD = new ParseField("replicated");
    public static final ParseField SYSTEM_FIELD = new ParseField("system");
    public static final ParseField ALLOW_CUSTOM_ROUTING = new ParseField("allow_custom_routing");
    public static final ParseField INDEX_MODE = new ParseField("index_mode");
    @SuppressWarnings("unchecked")
    private static final ConstructingObjectParser<DataStream, Void> PARSER = new ConstructingObjectParser<>("data_stream", args -> {