I tell artists that if they scan their originals and keep open the possibility of making prints in the future, it's best to let whoever buys the original know that this is a possibility.
Many people would be glad that their original had been turned into prints, but you never know how they'll respond if informed retroactively. It seems best to get a scan of your original before it ever leaves your hands, and to let people know up front.
We had something else happen several years ago that I was really uncomfortable with. I worked at a gallery here in town that sold limited edition landscapes by a photographer from out of town. Her work is lovely, and she's in the American Photography Hall of Fame, whatever that is. The editions were usually between seventy five to a hundred and fifty prints, and her work was popular enough that some sold completely out. That gallery closed in 1999 and I went to work where I do now. She followed me there and we began carrying her work. She lives about five hours away, and only came once a year. The next time she came, she had several of those earlier photos with her, but with no numbers on them. There are quite a few folks in town who bought them with number, thinking they were worth more as it was a limited edition, and I told her I wasn't comfortable selling more photos from those sold out editions. I know it's just photos and not something like hand-pulled lithos, but it still seemed like a betrayal of confidence.
From what I can see on her website, she's still selling those images with no numbers, and I feel sad about not carrying her in our town, but it just didn't seem right to do so. A person needs to either limit their edition and stick to it or not number it in the first place. I know this notion has been blown out of the water by certain well-known artist(s) in recent years, but it still matters.