When copying invoices you get an option to copy t 5 different headings, why not have sales copy to sales and purchase to purchase, may be have a button to request more options, but why would I want to copy a purchase to a sales invoice? he most likely thing is I had 20 invoices from one supplier and want to enter them.
On that note, can the date not copy as well rather tha constantly reverting to current? I have clients where the invoice come to me, often I will get 5 or 6 invoices in one envelope to enter, all with the same date a few days previously.