Here is a simple Sublime Text 2 plug-in to convert tabs (only at the beginning of lines) to spaces in the current file whenever you save it.

In Sublime, go to the Preferences menu and choose Browse Packages… This will open the Sublime Text 2 Packages folder. Create a new folder here with your last name, then open your new folder (you just created your own Package!)

Next, create an empty file in the folder called Paste this code into the file:

import sublime, sublime_plugin
# class ExampleCommand(sublime_plugin.TextCommand):
#   def run(self, edit):
#       self.view.insert(edit, 0, "Hello, World!")
class ConvertTabsToSpaces(sublime_plugin.EventListener):
      def on_pre_save(self, view):
      edit = view.begin_edit()
      view.run_command('expand_tabs', {"set_translate_tabs": True})
      #sublime.message_dialog("Converted endings.")

This creates a hook and runs whenever you save the file, but right before the save actually occurs, allowing you to modify the document text. In this case all we did was run the menu command expand_tabs. You could also run other commands or perform custom text replacements.