  Transition metal diborides crystallise in the $\alpha$, $\gamma$, or $\omega$
type structure, in which pure transition metal layers alternate with pure boron
layers stacked along the hexagonal [0001] axis. Here we view the prototypes as
different stackings of the transition metal planes and suppose they can
transform from one into another by a displacive transformation. Employing
first-principles calculations, we simulate sliding of individual planes in the
group IV-VII transition metal diborides along a transformation pathway
connecting the $\alpha$, $\gamma$, and $\omega$ structure. Chemistry-related
trends are predicted in terms of energetic and structural changes along a
transformation pathway, together with the mechanical and dynamical stability of
the different stackings. Our results suggest that MnB$_2$ and MoB$_2$ possess
the overall lowest sliding barriers among the investigated TMB$_2$s.
Furthermore, we discuss trends in strength and ductility indicators, including
Young's modulus or Cauchy pressure, derived from elastic constants.
