Spin-polarized scanning tunneling microscopy is used to study the magnetization reversal of individual quasiclassical and quantum atomic-scale magnets. Modifications of the dynamics arising from the presence of the biased magnetic probe tip are identified in terms of spin-transfer torque, Joule heating, Oersted field, magneto-electric coupling and spin-polarized field emission. Observing the switching behavior over a wide temperature range allows for a detailed study of nucleation, propagation and annihilation of domain walls in quasi-classical nanomagnets. Quantum magnets are found to be intrinsically stable at ultra-low temperature, and magnetization dynamics can be controlled by the spin-polarized tunnel current injection from the magnetic tip.