To anyone from that billing cycle for whom I had not already remedied the situation by resending (aka the clients who had already complained and brought this to my attention) I sent out a blanket BCC email with my humble apologies and an explanation (succinctly but definitively: QBO made changes without my knowledge) and offered to resend with the PDF attachment upon request. I would say about half of them requested this. One was a brand new client and I must have seemed unprofessional as heck!!! Fortunately, most of the others were repeat clients I've worked with month after month and year after year so the relationships are established enough to be understanding and cut me some slack. I can only imagine what a nightmare this could be for someone writing a hundred invoices a month for one off jobs or something similar in another field!