Potential source of hilarity;
The Culture give the Tau some similar form of rejuvination procedure.
The procedure repairs DNA and purges toxins.

The procedure turns out to purge the pheromones* used by the Ethereals to control the Tau castes, leading to older Tau to revert to their pre-ethereal, selfish and warlike tendancies and rebel against their mysterious overlords and their philosophy of the "greater good".

*Alternatively, simply extending their lifespan gives them time to build up a natural immunity. Or both!