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
* @return new {@code DataStream} instance with the added backing index
     * @throws IllegalArgumentException if {@code index} is ineligible to be a backing index for the data stream
     */
    public DataStream addBackingIndex(Metadata clusterMetadata, Index index) {
        
        final var parentDataStream = clusterMetadata.getIndicesLookup().get(index.getName()).getParentDataStream();
        if (parentDataStream != null) {
            if (parentDataStream.getDataStream().equals(this)) {
                return this;
            } else {
                throw new IllegalArgumentException(
                    String.format(
                        Locale.ROOT,
                        "cannot add index [%s] to data stream [%s] because it is already a backing index on data stream [%s]",
                        index.getName(),
                        getName(),
                        parentDataStream.getName()
                    )
                );
            }
        }
        
        IndexMetadata im = clusterMetadata.index(clusterMetadata.getIndicesLookup().get(index.getName()).getWriteIndex());
        if (im.getAliases().size() > 0) {
            throw new IllegalArgumentException(
                String.format(
                    Locale.ROOT,
                    "cannot add index [%s] to data stream [%s] until its alias(es) [%s] are removed",
                    index.getName(),
                    getName(),