Drop Ship Process doughts

 Hello All,

I have few question on drop shipments,Please clarify,they are as follows.

a) when we create a sales order and have purchase order created using purchase release.At this point how can we make changes to sales order.

However the order will not allow to make changes as PO will be sent for approval. Is there any way we can handle the situation where quantity on the sales order need to be increased.

b)  can we cancel the orders when the PO is created or sent for approval.

Thanks in Advance

Deepak