Well, I have found that shift-clicking on them in debug mode and selecting force aging and/or set to birthday will age them from adult, to elder, to puppy. I can't remember which one it was specifically that turned them into a puppy. Might try that, I suppose. Not sure it is a good thing to do, though. I was only doing it because my lot wouldn't let anyone age for some reason, and I got tired of having the dog around. Ended up having to give it away to a sim on another lot so it would age properly.
I know I did that to all my strays and they wouldn't age and all the big dog puppys had large donkey-like ears.