We describe a new kind of transition between topologically distinct $N=2$
type II Calabi--Yau vacua through points with enhanced non-abelian gauge
symmetries together with fundamental charged matter hypermultiplets. We connect
the appearance of matter to the local geometry of the singularity and discuss
the relation between the instanton numbers of the Calabi--Yau manifolds taking
part in the transition. In a dual heterotic string theory on $K3\times T^2$ the
process corresponds to Higgsing a semi-classical gauge group or equivalently to
a variation of the gauge bundle. In special cases the situation reduces to
simple conifold transitions in the Coulomb phase of the non-abelian gauge
symmetries.
