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
if (template.aliases() != null) {
                Map<String, AliasMetadata> aliasMeta = new HashMap<>();
                for (Map.Entry<String, AliasMetadata> cursor : template.aliases().entrySet()) {
                    aliasMeta.put(cursor.getKey(), cursor.getValue());
                }
                resolvedAliases.add(aliasMeta);
            }
        });
        return Collections.unmodifiableList(resolvedAliases);
    }
    /**
     * Resolve the given v2 template name into an ordered list of aliases
     */
    public static List<Map<String, AliasMetadata>> resolveAliases(final Metadata metadata, final String templateName) {
        final ComposableIndexTemplate template = metadata.templatesV2().get(templateName);
        assert template != null
            : "attempted to resolve aliases for a template [" + templateName + "] that did not exist in the cluster state";
        return resolveAliases(metadata, template);
    }
    /**
     * Resolve the given v2 template into an ordered list of aliases
     */
    static List<Map<String, AliasMetadata>> resolveAliases(final Metadata metadata, final ComposableIndexTemplate template) {
        if (template == null) {
            return List.of();
        }
        final Map<String, ComponentTemplate> componentTemplates = metadata.componentTemplates();
        return resolveAliases(template, componentTemplates);
    }
    /**