What about this? With the population growth such as it is, Congress decides the the 11 circuit courts are imbalanced, and decides to expand them to, say, 15. To do this they shrink the sizes of the 8th and 9th and create 2 more in each.
The cases can remain in whichever new circuit has jurisdiction over the district or state. But what happens to the displaced judges? Does one assume that a judge "redistricted" out of a smaller 9th automatically moves over to the new 12th, or is the new 12th a virgin court where the President is free to fill it up with fresh nominees?
-PJ
Which judges are retained gets determined by seniority.