The recently discovered superconductor UTe2 with a Tc between 1.5 K and 2 K, has attracted much attention due to indications of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36 T along the hard b axis in order to follow the superconducting transition up to the metamagnetic transition at Hm = 34.75 T. They demonstrate the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, possibly a spin-triplet to spin-singlet transition.