the big reason why you can not use the old hing slots is that the cas has hardened the wood and any new ca will not wick into the wood and create a strong bond. The ca you have on now might hold the hinges to the wood but there are not enough hinges to take the stress of flight and the hing itself might fail also the elevator having such large areas of unsupported length might start to flutter and rip the elevator off. when done properly ca hinghes are great but when done wrong its a horror story waiting to happen