Hi,
CDHDR and CDPOS will be much larger than the actual application tables themselves. As Andrea pointed out, they contain a change log for all objects in ECC. That means they will contain the changes of not just POs but also several other objects. So you will also have to implement logic somewhere to get only the PO tables by filtering on CDPOS-TABNAME = EKKO/EKPO etc.
These tables are not indexed in ECC, so expect very poor performance if you go down this road. Ideally I'd suggest to go with Gajesh's recommendation, however:
I need what field changed and it's before and after values.
This is very curious indeed. Can you elaborate further about what exactly is the requirement?
Regards,
Suhas