IMO as the whole point of Al Queda's actions was to keep hitting the USA until they succeeded in provoking them into a major reaction, in the hope that the US backlash would radicalise elements of the world's Muslim population (which is IMO exactly what has happened), they would keep trying until a plot succeeded.
So, sadly, instead of innocent American civilians dying in the New York on 9/11 others would die somewhere else in the USA on another date.
The interesting question to me is would they succeed before the US invaded Iraq anyway (as that invasion IMO had nothing to do with 9/11)?
If the Iraq invasion occurs before Al Queda manage to achieve a major attack on the US, they may decide there is no need for an attack as the invasion of Iraq will have the effect they are looking for.
Al Queda may be more powerful in this case, as they will have more popular support, and a secure base in Afghanistan. They may succeed in causing many more US casualties in Iraq and even effectively 'win' the conflict by pushing the US out of the cities.