We have a long-standing interest in the temperature- and magnetic field-evolution of magnetic and crystal structure in CoMnSi. This is driven by an interest in the magnetocaloric properties of the material around its metamagnetic phase transition. Using various magnetic and calorimetric data we have shown that this metamagnetic transition is tricritical. Our first, recent high resolution neutron scattering study on HRPD has revealed that this tricriticality probably comes from the field-induced coincidence of the metamagnetic phase transition with a hidden, zero-field structural change lying some 100 degrees below. We now wish to explore the opportunities for adjusting the tricritical point by tuning this hidden structural change, best evident through high resolution neutron diffraction.