Q.Christmas cactus
My Christmas cactus is 20 years old. The original stems are woody. What should I do, if anything?
Woody stems are part of the natural aging of a Christmas Cactus.
This actually indicates that your plant is doing well and aging well.
Here are some articles with more information.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/woody-christmas-cactus.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/advice-for-christmas-cactus-care.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/how-to-propagate-and-plant-christmas-cactus.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/christmas-cactus-pruning.htm