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
private final Map<String, Object> metadata;
    private final boolean hidden;
    private final boolean replicated;
    private final boolean system;
    private final boolean allowCustomRouting;
    private final IndexMode indexMode;
    public DataStream(
        String name,
        List<Index> indices,
        long generation,
        Map<String, Object> metadata,
        boolean hidden,
        boolean replicated,
        boolean system,
        boolean allowCustomRouting,
        IndexMode indexMode
    ) {
        this(name, indices, generation, metadata, hidden, replicated, system, System::currentTimeMillis, allowCustomRouting, indexMode);
    }
    
    DataStream(
        String name,
        List<Index> indices,
        long generation,
        Map<String, Object> metadata,
        boolean hidden,
        boolean replicated,
        boolean system,
        LongSupplier timeProvider,
        boolean allowCustomRouting,