I never do exactly the same technique twice. It will be contrary to aikido principles. As beginner you may be forced to change technique if the level of difficulty of attack is too high, but I'd say that this way one will never master techniques in very small details. And those details make whole difference between artist and artisan, and may be between life and death......

More one is advanced, less frequently he must change a technique, rather do small adaptation(directions, joints maniputation, distans, shifting weight vertically and horizontaly...etc.) and still preseving general shape of movement. That how one can learn all richeness of aikido.

Sugano sensei teach to observe what happens BEFORE contact. I'd say that if one master this aspect, technique is done before contact -- nage knows what and how he will react to attack.