$GLOBALS['TCA'] is built to allow to further manipulate the TCA.
Side note: It is possible to check against the original TCA as this is stored within $GLOBALS['TCA'] before this event is fired.
Method: getTCA() Description: Returns the current TCA (Table Configuration Array). ReturnType: array
Method: setTca(array $tca) Description: Sets (overwrites) the TCA. ReturnType: void