How to redevelop a production site? I'm working on a local version of an outdated Drupal 6 site which can not be shut down for a minute. The goal is to update the modules and give some enhancements, like adding tags, making a better menu and dynamic blocks etc with the least  possible down time for the site. 

It will probably take a few weeks for me to do so. Meanwhile new content will be added to the production site as per usual. 

My question is: When I finish developing the requested enhancements on local version, how should I integrate newly generated content to the development version before putting it online? I can not simply import the production database dump since this will undo my whole work. So obviously I need to import only certain tables. But how should I figure out which ones? What would you do in this situation?
It depends on what the enhancements you are making but the way I've done this same process before is to export the finished code improvements to a features module or other custom module from my dev site and use that to update the live site. 

You mentioned a few examples:


"adding tags" -> you could export the improved taxonomy and then import it on the new site using a number of modules, ex. taxonomy_xml or taxonomy_csv
"making a better menu" -> the features module allows you to export your menu and import on other sites
"dynamic blocks" -> easiest to just code these in a custom module (probably the simplest kind of module to write) and enable on the new site