dozy: It's probably about time that the circuits were renamed as the names dated back to the 50's when there were far fewer congregations so are often misnamed from a geographical standpoint
Ignoring the positive spin. I think the above would seem to be the real reason.
Circuits are traditionally named after the County they are in. Breaking out of this artificiality will enable circuits to be more efficiently organised, crossing over county borders, often enabling circuits to be better arranged for the CO to visit and for assemblies to be assigned.
The WT have, at last, realised they need to do a 'root and branch' exercise, rather than just fiddle with the boundaries, like they've normally done in the past.
Even without losing any congregations, it may be possible, due to more efficient boundary planning, enable them to reduce the number of circuits by 1 or even 2.