We have investigated the influence of controlled modifications of the surface electronic structure of Cu(001) on the spin polarization of photoelectrons emitted via multi-photon excitation. Using ultrashort, circularly polarized laser pulses with  ∼ 3 eV photon energy, spin-polarized electrons can be selectively excited from the spin-orbit (SO) coupled d-bands of Cu into the unoccupied n=1 image potential (IP) state on the Cu(001) surface. Upon lowering the IP state energy level by submonolayer Cs deposition, we show that the IP energy can be tuned into two-photon resonance with initial state d-bands of different double group symmetry, leading to a sign reversal of the spin polarization that is observed at the IP state level. Similarly, exploiting the parallel-momentum IP state dispersion, the resonant tuning of the IP state energy level to different branches of the SO split d-bands is demonstrated. Our results highlight the role of resonant and off-resonant excitation pathways in determining the spin polarization in the excited states. The additional information contained in spin-resolved multi-photon photoemission experiments can be exploited to obtain insights into the mechanism of population of excited states.