Both 还 and 又 may be used to indicate an addition to the preceding information:

我买了香蕉、苹果**还**买了葡萄。  
我们不但要努力学习，还要考得好。

昨天没考好，现在**又**撞伤了，真倒霉。  
他要买肉又要买鱼。

May I exchange both in all above examples? If not, what are the rules involved?

Thank you