Freezing of soft spheres in slit nanopores is investigated using Grand canonical Monte Carlo simulations. The pores are in equilibrium with a liquid located close to the liquid-solid coexistence region in the bulk Lennard-Jones phase diagram. In addition to layering, the confined fluid is found to possess in-plane order, leading to the formation of frozen phases which give rise to a sequence of solid-solid transformations as the pore width is varied. Transformations between n layered triangular to n+1 layered square lattices and between n layered square to triangular lattices, are observed for n=1, 2, 3, and 4. The transition from triangular to square lattices occurs via an intermediate buckled phase which is characterized by increased out-of-plane motion, while maintaining in-plane triangular order. Buckling was found to decrease with increasing number of layers. The transition between square to triangular lattices at a fixed number of layers is accompanied by a lowering of the solvation force, resulting in a doublet in the solvation force maxima. Influence of fluid-wall interactions on the nature of the frozen phases are studied by comparing the structures formed with a 10-4-3 and 10-4 fluid-wall potential. The solid structures are classified based on their closest 3D counterparts.