Needs company cars to be re-registered

Hello, I hope, here I get possibly a helpful answer, should another forum be able to help, please postpone. Now to my request: (incl. background history) We have a service company with fleet of 10 vehicles. Frima ran from 1999 to 2008 as an individual company on my mother. Company headquarters is in the district court with branch office among others in Stuttgart. 2005 we have 4 additional vehicles for our fleet and registered to the NL in Stuttgart t – the S-number gave in combination our Frimenname “SQS” and 1111, 2222 etc. On 1.1.2006 the registration regulation was amended so that vehicle can only be registered at the headquarters of the company – so we had to allow all vehicles purchased from this date in Hof. In 2008 the individual company changed to a GmbH. However, the NL in Stuttgart was not taken over and still runs today as a business of the private person (my mother). euge with registration in Hof were taken over into the GmbH and re-registered to the GmbH 2008 – the S-vehicles were not taken over and thus not re-registered. So and now to the own question: Last week we received from the Landratsamt Hof the notification that the S-vehicles must be re-registered to the yard, because the private person to which these are registered has their main residence in the district Hof ???? We also do not know why and how the registration office Hof after 4 years We would like to keep the S-numbers. Maybe someone knows good advice…. Thanks in advance DS