How do I add downloadable products to an existing order?

Opencart 1.4.9.1

I've had a couple of teething problems with downloadable products being corrupted, due to their size mostly. I have managed to resolve the problem with the downloadable files, however I haven't been able to grant customer the permission to download the new files I have uploaded and added to the store.

As soon as you edit a downloadable product, and change the file that the product is linked to (to another file), the customer doesn't have the rights to download that file.

I have tried removing the original product from the customer's order, and adding it again (now with the new file), but when I do that - or add any other product with a downloadable file, the customer doesn't have the option to download the new files in the downloads section of their account page.

Is there anyway around this?

Any advice on this one guys? Not really sure where else I can ask the question.