A method for installing a compressor and a gas turbine of a first type at a position of an existing power plant where previously a compressor and a gas turbine of a second type were installed on a foundation specially designed for the second type. The two types differ from each other at least with respect to the position and/or the number of anchoring points at which the compressor and the gas turbine are connected to the foundation via support structures. The installation of the new compressor and the new gas turbine is carried out using an adapter structure on the existing foundation. A corresponding adapter structure is provided.