  We prove a universal substitution formula that compares generating series of
Euler characteristics of Nakajima quiver varieties associated with affine ADE
diagrams at generic and at certain nongeneric stability conditions via a study
of collapsing fibres in the associated variation of GIT map, unifying and
generalising earlier results of the last two authors with N\'emethi and of
Nakajima. As a special case, we compute generating series of Euler
characteristics of noncommutative Quot schemes of Kleinian orbifolds. In type A
and rank 1, we give a second, combinatorial proof of our substitution formula,
using torus localisation and partition enumeration. This gives a combinatorial
model of the fibers of the variation of GIT map, and also leads to relations
between our results and the representation theory of the affine and finite Lie
algebras in type A.
