How to convert individual invoices to recurring invoices

[Ops11]

Hi. I have several hundred invoices that were sent as one-time invoices. I now want to update all of them to be recurring invoices so a new invoice is automatically sent to each customer on an annual basis. My questions are:

1) How do I edit an invoice that's already been sent (and paid) so that it becomes a recurring invoice?

2) Is there a way to do #1 in bulk?

BTW, using the PayPal API is acceptable if this is the only way to do it.

Thanks!