Which elder arrangement are your referring to?
Prior to 1972 or 1973 there was a 'Congregation Servant' (I think). It was just one man overseeing everything in a congregation.
In 1972 or 1973 an arrangement was created for each cong to have a 'body of elders' rotate thru 5 different roles in congregation oversight (Presiding Overseer, Service Overseer, Secretary, Theocratic School Overseer and Literature Overseer).
The most recent change that I know of involved appointment of a CoBE (Coordinator of the Body of Elders) but that happened LONG after I left the org and I do not know how or why that works.
I do know that shortly after the CoBE arrangement, there seemed to be a deletion of elders who had served for a very, very long time. But I do not know if/how the CoBE thing specifically had anything to do with the deletions and purges.