What happens when an electron in a metal is excited?
You seem to be misunderstanding what is a "sea of electrons". In fact, this is a metaphor upon a metaphor upon an abstraction. There is no sea. There is a huge bunch of orbitals. (Sure, the solid state people prefer to call them "states", but that's not really important.) The whole piece of metal is a giant molecule. It is not all that different from ordinary small molecules, except that it is very big, and many orbitals span the entire molecule (but then again, that's what they often do in normal molecules).

All these orbitals tend to have different energies. They are everywhere on the energy scale, very close to each other. You point your finger at any given energy, and you find an orbital with that energy. We can't really tell them apart. They kinda blend into a continuous spectrum. And that's what we metaphorically call the sea of electrons.

Electrons are not free, they are confined to some states. When a photon hits, any electron can get excited all right. It will move up to one of the empty states. In fact, it has an immense freedom compared to an electron in a small molecule. Instead of a few discrete states, it has an entire sea of them above. It can get excited to any of them, which in particular means that metal as a whole can absorb a photon of any energy. This, BTW, is also an explanation of metallic luster.

The excited electron is not always the valence electron (you may fire an X-ray photon at a substance and excite an electron from core orbitals deep down below), but that's irrelevant.