You need to upgrade when moving from the Camilian to Polybian and when moving from the Marian to Augustan. When moving from the Polybian to Marian, new MICs will be placed in your settlements of equal size to the ones you currently have.

And as always, if you reach the highest level of MIC and reach a reform, your MIC will be able to upgrade from highest level Camilian/Marian to the highest level of Polybian/Augustan.