@BinaryCats:
Yeah I know what you mean, it's such a useful feature it would be great if it was built into the editor, even if that would lock out users of older versions of the editor (due to 3rd party runtime compatibility etc).
An external tool which allowed the user to choose a version of Spine from a dropdown list, and merge skeletons from that version would definitely have my money! (hint hint lol!)

@Macguffin:
Ahh that's interesting, hopefully someone "in the know" can comment on this thread soon and clear things up, but that looks promising.