I'd say if interchangeable, it would HAVE to be even tiller so it would shoot right.
Also, the limb alignment for smalltime bowyers that build one at a time would be a prob,
you would have to have some precise equipment to get the limb to bolt on and line up.
Not a prob for guys with CNC and such, but a real headache for me!!
For most folks, it is just pick the weaker limb for top , I would venture to say.