This section is turning yellow, the rest (upper) sections are still green. Why is it doing this and what do I do?

It's a sign that that particular stem is rotting/dying from loss of roots. I would cut off and discard the yellowing portion at the joint where the stem is still green, otherwise it will continue to rot the whole stem. Let the top portion dry out for a couple of days to allow the end to callus and then replant it. Someone else will come along soon to verify if this is the case or have a better recommendation.

I was thinking along those lines, want to save the plant. I need to replant it anyway, the soil it is in needs replacing and that pot doesn't have any drainage.

You should not be growing succulents ( or any house plant) in a pot with no drainage. Gene

yes I realize that, I am repotting them.

Good for you. It's a nice specimen, and I'd hate for you to lose the plant.