rules autotag module, bulk retagging existing nodes
According to this autotag issue on d.o, titled "How to re-tag all content":


  
  Use Views Bulk Operations (VBO).
  Create a bulk view with the 'Save post' operation
  In the view, select the nodes you want to re-autotag and execute (which will resave all the nodes)
  


Note the different autotag modules:


autotag uses your existing taxonomies, can be used with VBO.
rules autotag doesn't have the UI of autotag, it's possible to use this one with VBO as well.
autotagging uses 3rd party services