How to change PO in case of change in PR

Hello Experts,

My requirement is to create and change PO in case PR is created and changed.

If PR is created PO needs to be create.

for this i am using

FM <BAPI_PO_CREATE1>

i got this.

If anything is changed in PR ( Eg. Qty, material description or anything...) PO must have to change with respect to PR.

Note : I even dont know which fields can be changed in PR.

Please Help.

Thanks in Advance.