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
CollectionUtil.timSort(matchedTemplates, Comparator.comparingInt(IndexTemplateMetadata::order).reversed());
        
        
        if (isHidden == null) {
            final Optional<IndexTemplateMetadata> templateWithHiddenSetting = matchedTemplates.stream()
                .filter(template -> IndexMetadata.INDEX_HIDDEN_SETTING.exists(template.settings()))
                .findFirst();
            if (templateWithHiddenSetting.isPresent()) {
                final boolean templatedIsHidden = IndexMetadata.INDEX_HIDDEN_SETTING.get(templateWithHiddenSetting.get().settings());
                if (templatedIsHidden) {
                    
                    matchedTemplates.removeIf(current -> current.patterns().stream().anyMatch(Regex::isMatchAllPattern));
                }
                
                final Optional<IndexTemplateMetadata> templateWithHiddenSettingPostRemoval = matchedTemplates.stream()
                    .filter(template -> IndexMetadata.INDEX_HIDDEN_SETTING.exists(template.settings()))
                    .findFirst();
                if (templateWithHiddenSettingPostRemoval.isEmpty()
                    || templateWithHiddenSetting.get() != templateWithHiddenSettingPostRemoval.get()) {
                    throw new IllegalStateException(
                        "A global index template ["
                            + templateWithHiddenSetting.get().name()
                            + "] defined the index hidden setting, which is not allowed"
                    );
                }
            }
        }
        return Collections.unmodifiableList(matchedTemplates);
    }
    /**