Modify vb.net 2008 Dataset

Question

I have an existing vb.net 2008 application where I need to modify several datasets using the designer. Basically I am making this change since some of the tables accessed by this application will be changing.
Thus my question is, is it better to modify the exsiting datasets or is it better to generate new datasets and delete the old datasets? If so, can you tell me why one method is better than another method?
Also I wanted to mention for the existing application, there are some mthods that are used that 'extend' the features of the dataset base code.