In general, if you can't do it to one record through the front end, you won't be able to do it using the data entity via any backdoor. There are lots of different reasons why a PO Line might not be editable, some of which will apply to you, and I guess some which won't.... but if you can find a way to update one of these historic POs through the front end, then get the other historic POs in to the same state and you'll be able to update them using the data entity.
That said, if the PO Line has an Item number on it, you probably won't be able to update the Category; I think that's how F&O works. You might need the help of a developer; perhaps they can set the Category on old PO lines by referring to the category which the item is currently assigned to.
