Your post is unclear....management has the right to realign jobs or change jobs or shifts or just about anything ..absent a collective bargaining agreement to the contrary or perhaps,some prevailing wage issue if such applies.

If the older worker lacks the required skills or productivity of the new assignment ..the older worker has a big problem and management is free to let him go.....that's life.

Where it gets problematic is if person over 40 is singled out primarily for age reasons for some unfavorable treatment compared to younger workers. But that is not what you seem to post.