Again, the general rule here is to try not to change what comes with the Purchase Order system. You can move fields about to some extent with out problems. Be careful of the order of validation fields and their lookup fields.
If there is a field on the screen that you do not use, you can make the field invisible. This approach is preferred over removing the field because the user edit routine that goes with the screen, may try to read or write information to the field.
As before, adding fields to the screen is usually no problem. You do need to watch out for the validated fields and their lookups, but that should be the extent of any problems you might encounter.