Question Changing parts of character model in runtime

Hey guys, i'am working on some project lately and I want to know how would you handle problem I encountered. Let's say that we have some model (monster, robot etc) which is divided into different parts (head, chest, right arm, left arm, right leg, left leg). This model have it's own mesh, rig and animations connected to that rig.

I want to change parts of this model in runtime, for example change one head for some other one or chest with other chest, etc. (something like in game "Spore" but of course much simpler). Parts will be different from each other: new head or arm could be bigger than the previus one or bottom body of a model could be changed from two legs (humanoid legs) to four legs one (like: take bottom part of human, change it to horse legs and you have centaur), and all of this while keeping the same Rig.

How would you approach such a task? I would appreciate any help in this matter.