You can do this by setting Document Number of PO to manual.
[A] Go to "Settings Module", Scroll below to "Document Number Format".
[B] In "Document Number Format", set "Purchase Order" number to "Manual" from Automatic.
[C] Enter the same old PO number while making PO, save it.
[D] Once you save it, change the Document Number of PO to Automatic again.
Note: This steps will only help you to enter old PO number again, but the current date will always remain for the document.
Keywords: PO, Document Number retain