The thermoreversible self-assembly of poly(methyl methacrylate)-b-poly(tert-butyl methacrylate) (PMMAPtBMA) diblock copolymers in 2-ethylhexanol has been studied in the transition regime between spherical and cylindrical morphologies. In these materials the PMMA block exhibits a strong temperature dependence of the solvent quality that leads to reversible micelle formation, and the PtBMA block is a versatile polymer that can be hydrolyzed for further use as a polyelectrolyte. Self-consistent field theory was used in combination with a variety of experimental techniques to develop a simple criterion for the location of the sphere/cylinder transition in solutions with concentrations above the micelle overlap threshold. It is shown that the effective volume fraction of PMMA core, accounting for solvent swelling of the micelle core, is equal to approximate to 0.27 at the sphere/cylinder transition. For the spherical domain morphologies, a transition between disorded micelles and micelles packed on a body-centered-cubic lattice occurs when the micelle hydrodynamic radius of the micelle is comparable to the intermicelle spacing in the ordered micellar structure.