TextMate : Keeping my KeyBindings after an update
Under MacroMates' keyBindings page is written:


  In addition TextMate has a /path/‍to/‍TextMate.app/‍Contents/‍Resources/‍KeyBindings.dict file with some extra key bindings which are specific to TextMate (and thus not appropriate to put in the per user global key bindings file). You can copy this file to ~/Library/‍Application Support/‍TextMate 
  and edit it, this will then take precedence over the bundled file.


As this file won't be replaced during the Application's update, your keybindings will remain.

Note: As Matthieu points out, you will need to rename the file to DefaultKeyBinding.dict to make it work.