Helllo Peter, all,
Thorn CarpetRegrid2 can be "steered" at run time by modifying its grid scalars (NOT its parameters!). You will then need to write a thorn to e.g. set active[0] and active[1] to zero.
Frank Loeffler's Trigger thorn can do this based on eg time or density maximum or other quantities. See https://docs.einsteintoolkit.org/et-docs/Thorns_we_know_of
Yours, Roland