Thanks for your question.

As you know, Ti plants are native to Hawaii.  Consequently they are used to warm and rather humid conditions.  These conditions are hard to duplicate  inside during our Minnesota winters.  However since apparently you have had this plant for 50 years, under presumably the same interior conditions, I do not suspect that light and/or humidity are responsible for the current condition of your plant.  The leaves showing yellowing and darkening are probably the oldest leaves on this plant so their appearances are not completely unexpected.  Yellowing of leaves is usually indicative of chlorophyll loss which in turn signifies some plant stress is occurring.  My suggestion would be to remove these lower leaves but not immediately close to the stem.  Leave about a half inch or so of the leaf’s stem.

If the soil in the pot feels damp to the touch, hold off on further watering until this dampness is gone.  Consider misting the plant instead of direct watering.  The humid conditions that Ti plants enjoy are best achieved through misting than watering.  You said that transplanting did not seem to improve the situation.   However I would suggest that you do this again but into a larger pot.  This will promote better root development and nutrient uptake.

Application of some liquid fertilizer such as Miracle-Gro might be another good step. 

The following publication will give you information of these and other points: 

https://www.gardeningknowhow.com/houseplants/ti-plant/growing-hawaiian-ti-plant-indoors.htm

Good luck!!