Hetzner desteğe yazdım cevap geldi. mümkün değilmiş. birilerinin işine yarar diye gelen mesajı paylaşıyorum.

" thanks for your message.

Unfortunately it is technically not possible to shrink a Cloud-Server to a model with a smaller disc. This comes due to overlaying filesystems. If you wish to scale down to a smaller model, this would need to be done manually, by migrating respective data and software by hand."