[quote]Any particular reason for this change?[/quote]
It's more rigorous from a code viewpoint.
In W2012, using a parameter block on the layout or defining a new parameter in the expression for the weight of a transformation, would both result is a parameter OBJ in the MSL code of the (de)Fractionation model.
In W2014, a parameter can only be defined in one way, i.e. at the level of the expression for the weight of a transformation.
[quote]How does it work for using WEST 2012 fractionation models in WEST 2014?[/quote]
Until you don't regenerate (code and influent files) it should work just fine - as what matters is the MSL code and there, how a parameter OBJ was created, is of no relevance.
If you need to regenerate, it may be necessary to first update the W2012 layouts to the new "concept".