I would not call it a retcon..it's just their origins were fleshed out more. Especially since it just made sense since they are the souls of people who went to hell. Just like ghosts are the souls of people who didn't go to heaven or hell.
I did take issue with the whole "destroy their bones" thing working on demons. For me it made more sense for ghosts because they were sort of bound here and didn't want to leave. It also makes me wonder if you sold your soul and then when you are killed by hell hounds..if they salt and then burn your bones do you still go to hell?
Also notice people sell their souls for really shitty things? At least ask for super powers, not to be really good at being an architect or something.