When you deploy something to a site, you will most probably use features. To use featutes with hook_update_N you can use default
features_revert(array('feature_name' => array('feature_component', 'feature_component_2')));but you need to name all of the components and keep track what is changing. To make it in a bulk and check all components, you can add one helper function.
https://gist.github.com/cam8001/1925886
or
http://definitivedrupal.org/erratum/reverting-features-update-hooks-req…
both work, do the same thing, first does it in more elegant style. If you don't use this, you will need to examine features info file and put proper feature component name, which can get very big if you are exporting for example content types.
UPDATE:
There is now features_revert_module function which does the same as above, so no need to add helper functions.