You need to verify that you can move (and remove) the electrical line to the light, before worrying too much about the replacement.

Pool lights have a long waterproof cable that extends to an above-ground junction box. It's quite common for these to be installed badly, and to become permanently stuck as a result. Short of major excavations, there's no real fix if this is the case with your light.

Some pool guys have become adept at removing these, using a combination of lubricant and cable pullers . . . but there are no guarantees.