In BBEdit, Mailsmith, or TextWrangler, choose Scripts from the Palettes submenu of the Window menu. Click on BBAutoComplete in the palette that opens, and then click the Set Key... button.
In Script Debugger or Affrus, choose Show Scripts from the Palettes submenu of the Window menu. Click on BBAutoComplete in the palette that opens, and then choose Set Keystroke... from the down-pointing triangle menu opposite from the Edit button.
In Smile and Tex-Edit Plus, you can change the key by modifying the letters at the end of the script file’s name. For the details of how this works, please consult the Smile and Tex-Edit Plus documentation.