this is still an issue in OpenSim. So, you have to do it "manualy".
First of all, please shut down your local simulator. Afterwards, use our Region-Management https://metropolis.hypergrid.org/regioman.php
to delete the old region entry in our database. Be sure to login with the name of the estate owner. As the last action please restart your local simulator. The new database entry with the new region name will be written automaticly.