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/MetadataIndexTemplateService.java
final Set<String> newlyUnreferenced = findUnreferencedDataStreams.apply(updatedMetadata);
        
        
        if (newlyUnreferenced.size() > currentlyUnreferenced.size()) {
            throw new IllegalArgumentException(
                "composable template ["
                    + templateName
                    + "] with index patterns "
                    + newTemplate.indexPatterns()
                    + ", priority ["
                    + newTemplate.priority()
                    + "] "
                    + (newTemplate.getDataStreamTemplate() == null ? "and no data stream configuration " : "")
                    + "would cause data streams "
                    + newlyUnreferenced
                    + " to no longer match a data stream template"
            );
        }
    }
    
    private static void validateTsdbDataStreamsReferringTsdbTemplate(
        ClusterState state,
        String templateName,
        ComposableIndexTemplate newTemplate
    ) {
        Metadata currentMetadata = state.getMetadata();
        Metadata updatedMetadata = null;
        Set<String> dataStreamsWithNonTsdbTemplate = null;
        for (var dataStream : state.metadata().dataStreams().values()) {