  Techniques of producing new combinatorial structures from old ones are
commonly called trades. The switching principle applies for a broad class of
designs: it is a local transformation that modifies two columns of the
incidence matrix. In this paper, we present a construction, which is a
generalization of the switching transform for the class of Steiner 2-designs.
We call this construction paramodification of Steiner 2-designs, since it
modifies the parallelism of a subsystem. We study in more detail the
paramodifications of affine planes, Steiner triple systems, and abstract
unitals. Computational results show that paramodification can construct many
new unitals.
