There are a few documents available to achieve this kind of requirement. However, those are all dedicated to applications 11,12 and 13. I think SAP do not provide a user exit to record the changes to purchase orders as it does provide for above applications MCS10001.
However, you can take a look at few SAP notes that can help you.
Refer below which mentions the same thing but unfortunately refer only above 3 application areas.
1850433 - Change to user-defined fields using LBWE are not extracted in the delta
With this you can read from logging: (not a very good solution in my opinion 
)
1760322 - Activate log file for delta extraction, application 02, 03, 04, 08, 11, 12, 13
You might have to create a new datasource on change long tables and integrate it in BW system with the data.
Thanks
Amit