Christmas cactus would grow in the crotches of tree in their native environment so do’t need much soil. 45 years is a long time in a pot though. Don’t jump to a much larger pot. Pick one that is only an inch or two larger and make sure it has a drainage hole in the bottom. Make sure no to bury the cown of the plant…just add soil around and under the roots but none on top of the existing soil. Regular potting soil is fine. Late winter early spring is the best time for this project,