I notice that it is accepted practice to perform geometry optimizations and frequency calculations at one level of theory in DFT (using a particular functional/basis set combo), and to then use this optimized structure to perform single point energy calcs and TD-DFT calculations at a different level of theory.

Is this practice technically correct, or is it a big approximation that has just become the norm? Because the way I see it, every level of theory produces its own Potential Energy Surface (PES). Maybe I just don't understand enough, but once you change anything, like functional, or basis set, or solvent model, you will be generating a totally new PES, right?