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.
modules/data-streams/src/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
}
                var templateName = "__dynamic__" + template.name();
                var mappingSnippet = template.mappingForName(templateName, KeywordFieldMapper.CONTENT_TYPE);
                String mappingSnippetType = (String) mappingSnippet.get("type");
                if (mappingSnippetType == null) {
                    continue;
                }
                MappingParserContext parserContext = mapperService.parserContext();
                var mapper = parserContext.typeParser(mappingSnippetType)
                    .parse(template.pathMatch(), mappingSnippet, parserContext)
                    .build(MapperBuilderContext.root(false));
                extractPath(routingPaths, mapper);
            }
            return routingPaths;
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }
    /**
     * Helper method that adds the name of the mapper to the provided list if it is a keyword dimension field.
     */
    private static void extractPath(List<String> routingPaths, Mapper mapper) {
        if (mapper instanceof KeywordFieldMapper keywordFieldMapper) {
            if (keywordFieldMapper.fieldType().isDimension()) {
                routingPaths.add(mapper.name());
            }
        }
    }
}