Magento module to use "Observer" instead of "Rewrite"

1) An old module is using <blocks> <adminhtml> <rewrite> 2) Another module which is using obverser class to display a button but since the first module is rewriting the same block, it is never shown. What needs to be done is that the first module needs to start using observer class which is the better way of adding buttons Only experience guys would know what I am talking about.