I am presently studying Nathaniel Dett's Chariot Jubilee, and the time signature switches from 2/2 to 4/2. What are some suggestions on how to approach explaining to singers.
A switch from 2/2 to 4/2 is actually quite easy.
Since they both have a beat unit of one half note (that's the 2 on the bottom of the time signature), the only thing that changes is that a measure of 4/2 has 4 half notes in it instead of just the 2 found in 2/2.
In fact, you can basically tell your singers to just keep thinking in 2/2. If they mentally insert a bar line every 2 half notes, it's as if the time signature never changed.
(In fact, it's such an easy switch that I'm wondering if there's a typo in your question...)