Copy rows from the same table and update the ID column

INSERT INTO ProductDefinition (ProdID, Definition, Desc)
SELECT
  xxx, Definition, Desc
FROM
  ProductDefinition
WHERE
  ProdID = yyy

The xxx is your new ProdID and the yyy is your old one. This also assumes that DefID is automagically populated on INSERT.

This will work with any column you choose. Not just primary key/ID.

INSERT INTO TableName (Column1, CustomID, Column3, Column4, Column5)
SELECT Column1, 'NewValue', Column3, Column4, Column5 FROM TableName
WHERE CustomID='OrigValue'