Multi-photon laser-lithography has become a versatile technology for writing complex 3D structures in high-resolution. Most efforts are focusing on single material printing. To realize multi-material printing, two main steps have to be carried out consecutively: developing the previous material and introducing the next material to the print side. So far, this is done ex-situ, and alignment errors are unavoidable. In this work, a fast in situ material exchange is demonstrated. The designed setup can effectively replace the print material in the optical focus, enabling the writing of detailed 3D structures while conserving efficiency and precision.