By means of hybrid density functional theory we investigate the evolution of the structural, electronic and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO$_3$ under pressure. We predict a transition from a low pressure antiferromagnetic (AFM) insulator to a high pressure ferromagnetic (FM) transport half-metal (tHM), characterized by a large spin polarization (~ 80-90 %). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the $Pnma$ orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S=2, m_Mn=3.7 mu_B) to low-spin (S=1, m_Mn=1.7 mu_B) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite.
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