"so as to = so that ... may " 表示目的，而"so ... as to = so ... that" 表示结果，因此例句可分别改写成: 

a) We started so early that we got there before noon. 

我们动身很早，所以在正午之前就到达那里了。 

b) We started early so that we might get there before noon. 

我们要在正午之前赶到那里，所以很早就动身了。 

     又如: 

He was so angry as to be unable to speak. 

(He was so angry that he could not speak.) 

他大为生气，连话都说不出来了。 

Come early so as to get a good seat.

 (Come early so that you may get a good seat.) 

早点儿来，可以站到好位置。