Changling?

What are "Changling"? Supposedly Multiple man is a Changling, but I'm still confuse on what is it.

I think it's a level of shapeshifting ability. For example morph is a megamorph, a much higher level.

But, what does Shapeshifting have to do with duplicates?

Predecessors to mutants who develop their powers at birth. Damian Tryp suggests that Jamie and himself are throwbacks to this earlier breed of mutant.