I just rushed through that tutorial - and as far as I can tell that "transformation" is nothing more than a frame-animation. While the artist there used flash to draw the single frames. Maybe there are some tools that make it easier for him.
Frame animation IS possible in spine. So, yes, spine is capable of doing something like that - except the drawing within the tool. You will have to draw each single frame with a drawing programm (e.g. photoshop). As long as it is just for a small animation - say one before the level starts (as you mentioned) - this is no big deal. If you might want to morph a fully spine animated ingame character it might take a litte bit more effort. Though it is possible .
I think the idea of a morphing character is very interesting and I'm quiet motivated to do a small test/example animation now. But actually I'm at work right now - so I have to do other things :p and I'm a littlebit short on time the next week. This means you might have to wait some days untill I could show one, sorry.
