Thankyou so much for your detailed reply.

I have fixed the issue by using the same procedure you also highlighted in your reposne.

Thankyou once again ! 

First of all you cannot cancel "PO" if you have validated any quantity of it.

There are 2 scenarios in this.
First of all, if you are using "Purchase Control Policy" as "Received Quantities", then as you have only received 2 units and you can validate it by accepting 2 quantities and when Odoo ask you that you have to receive 9 however you are receiving only 2, and it shows below some buttons , use "No Backorder" Button. Now when you will come on "PO" and create Bill only 2 quantities Bill will be Charged. Then you can create new "PO" for new items.
If mistakenly you have created "Backorder" of remaining quantities, you can just "Cancel" it and you will be able to create bill of only 2 received quantities.Then create new "PO" for new products.

Now if you are using "Ordered Quantity" Policy and you have already created and paid Bill for that 9 items.And also when you received 2 products you have created a Backorder. Simply you can edit your "PO" and make Product A Quantity to 2, and add Product B in list and make it quantities to 7. Now if they are same price make price same as Product A and Save it , Now previous backorder of 7 remaining quantities will be cancelled and new Receipt will be created for receiving Product B.
After receiving product B, "Create Bill" Option will be shown on "PO" click on it and check the price of Product B is same as Product A on Bill as well. Because this will revert 7 quantities of Product A and add 7 quantities of Product B and your overall total Bill will be zero because you already have paid it, simply confirm bill and it will be marked as "Paid".

Hope you understand it.
Don't hesitate to ping me up if you don't get it because at first time it may look confusing.