Moving fields: I would like to move a field from the PO Header Details screen to the first screen on PO Creation. Is this feasible?

This is possible in most cases, but it will require some InputScript programming. Here are the steps you have to implement:
Define a new entry field on the first PO screen, copying the label text and field length from the field you want to move. If there is a special search help (match code) for the field, please use the "TechnName=..." parameter of the InputField statement.

 Sometimes only a few values are actually allowed as possible input values. In this case, a couple of radiobuttons might be a good solution, instead of a new entry field.

Next you have to find the best way to carry the entered value into the standard SAP field (which you can delete from screen). This can be done in an InputScript that you start with "On Enter" or "On /xx" on the first screen. You navigate to the screen with the standard field, set the value, and then go to the first screen that the user should see in the transaction. Sometimes it is not possible to navigate to the screen with the standard field, without making an input in some obligatory fields. Either you can put these required fields on the first screen as well; or you set the new value at a later point in time, e.g. when the user presses the "Save" button.