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 List<String> missingComponentTemplates = template.composedOf()
            .stream()
            .filter(componentTemplate -> componentTemplates.containsKey(componentTemplate) == false)
            .toList();
        if (missingComponentTemplates.size() > 0) {
            throw new InvalidIndexTemplateException(
                name,
                "index template [" + name + "] specifies component templates " + missingComponentTemplates + " that do not exist"
            );
        }
    }
    public ClusterState addIndexTemplateV2(
        final ClusterState currentState,
        final boolean create,
        final String name,
        final ComposableIndexTemplate template
    ) throws Exception {
        return addIndexTemplateV2(currentState, create, name, template, true);
    }
    public ClusterState addIndexTemplateV2(
        final ClusterState currentState,
        final boolean create,
        final String name,
        final ComposableIndexTemplate template,
        final boolean validateV2Overlaps
    ) throws Exception {
        final ComposableIndexTemplate existing = currentState.metadata().templatesV2().get(name);
        if (create && existing != null) {
            throw new IllegalArgumentException("index template [" + name + "] already exists");
        }