Google Cloud Platform: Hardware configuration cannot be changed in a pre-enterprise GCP VM

Today I tried to change the hardware configuration of some preferred instances in us-east1-d to include a single GPU (T4) each. When I applied the changes, GCP throws the following error:

The request contains invalid arguments: "(1-24) vCpus can be used together with 1 accelerator cards of type & # 39; nvidia-tesla-t4 & # 39; in one instance". Error code: "badRequest"

The instances use 24 CPUs. I am able to create new instances with the destination configuration without any problem.

Edit:
Adding a T4 works when you configure the CPUs in 24 separately before in an additional editing step. However, changing the configuration to two K80 GPUs does not work even when you configure the machine with 16 CPUs before in an additional step:

The request contains invalid arguments: "the characteristics (n1-standard-16, nvidia-tesla-k80) are not compatible to create the instance". Error code: "badRequest"

Thanks in advance.