Most (read ALL) lights have a flooded conduit, unless ground shifting/ freezing has compromised the conduit, in which case, someone may have puttied the niche to stop the leak and you'll have to remove the putty to pull the old cord out and install the new one, and then re-putty the niche when you install the new light.

The wires are very simple, the black one carries the current, the white one is neutral and the green one is the grounding wire. At your junction box, just make note as to which color wire attaches where and replicate that when you install the new cord. (but you sound like you know electricity far, far better than I)

When I pull a light cord, as you suggested, tie a string to one of the old cord's wires (and tape it) and pull the old cord out. If it's tough to pull the old cord out, either the electrician used a small conduit or the conduit has broken somewhere and the broken pieces are pinching the cord .

When going to install a new light, I unravel the cord - I walk around the pool 'unwrapping' the cord, so that there are no 'kinks' to get stuck at the back of the niche, where the conduit attaches

It helps to have someone 'feed'/ force the cord, if it get's stuck (YOU DO NOT WANT TO BREAK THE STRING!!) A little lube on the first couple of feet of the cord helps.

Maybe I helped with this, maybe I didn't - if you need more of my advice, simply ask