That's simple, you can't.

Have you thought about using an optional field.

when somebody changes the PO, they go into the optional field and changes it to the next revision.

On the PO form, just add this optional field after the PO number - don't forget to left justify the revision number and right justify the PO number.

Thanks for the replies.

Naturally we thought about Optional Field first, but it is not practical. People tend to forget to change the revision number manually. We need an automated solution.

Use an optional field and a database trigger on POPORH1.UPDATE.

Would like to have a solution without using Optional Field.

There are several "Date" fields in PO header, trying to figure out a way by using "Last Post Date", or something similar to trigger the "revision number".

Any suggestions guys?

Never screw with dates, you'll corrupt the data, maybe even prevent a dump/load.  Find another unused field, like the 4th address line, and take that line off the printed forms.