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
public ClusterState execute(ClusterState currentState) throws Exception {
                    validateTemplate(request.settings, request.mappings, indicesService);
                    return innerPutTemplate(currentState, request, templateBuilder);
                }
            },
            ClusterStateTaskConfig.build(Priority.URGENT, request.masterTimeout),
            TEMPLATE_TASK_EXECUTOR
        );
    }
    
    static ClusterState innerPutTemplate(
        final ClusterState currentState,
        PutRequest request,
        IndexTemplateMetadata.Builder templateBuilder
    ) {
        
        
        boolean isUpdate = currentState.metadata().templates().containsKey(request.name);
        if (request.create && isUpdate) {
            throw new IllegalArgumentException("index_template [" + request.name + "] already exists");
        }
        boolean isUpdateAndPatternsAreUnchanged = isUpdate
            && currentState.metadata().templates().get(request.name).patterns().equals(request.indexPatterns);
        Map<String, List<String>> overlaps = findConflictingV2Templates(currentState, request.name, request.indexPatterns);
        if (overlaps.size() > 0) {
            
            if (isUpdateAndPatternsAreUnchanged || request.indexPatterns.stream().anyMatch(Regex::isMatchAllPattern)) {
                String warning = String.format(
                    Locale.ROOT,
                    "legacy template [%s] has index patterns %s matching patterns"