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 indexMode
    ) {
        this.name = name;
        this.indices = List.copyOf(indices);
        this.generation = generation;
        this.metadata = metadata;
        assert system == false || hidden; 
        this.hidden = hidden;
        this.replicated = replicated;
        this.timeProvider = timeProvider;
        this.system = system;
        this.allowCustomRouting = allowCustomRouting;
        this.indexMode = indexMode;
        assert assertConsistent(this.indices);
    }
    private static boolean assertConsistent(List<Index> indices) {
        assert indices.size() > 0;
        final Set<String> indexNames = new HashSet<>();
        for (Index index : indices) {
            final boolean added = indexNames.add(index.getName());
            assert added : "found duplicate index entries in " + indices;
        }
        return true;
    }
    public String getName() {
        return name;
    }
    public TimestampField getTimeStampField() {
        
        return TIMESTAMP_FIELD;