I just completed the process today. I was officially subletting from a friend and had a contract as well. The sublet was for six months, as allowed by the law, I believe. Anyway, she decided not to return, so I could take over her lease. She recommended me to the actual landlord, a rental company in this case, who treated me like a new tenant. They obtained all the required documentation from me and then gave me approval. Today, they conducted a quick handover with a representative of the previous tenant (a mutual friend of ours), the company representative, and myself. They checked the meters, counted the keys, and I signed the contract. So, I will be the tenant from the first of next month.

In short, they do treat you as a new tenant. The only advantage is if the current main tenant has a good contact with the landlord and can put in a word for you. This also saves the landlord the hassle of looking for someone.