We consider the construction and implementation of a 'free-Lagrange' finite element method, where local mesh modifications can be introduced during computation. The method is based on the use of space-time oriented meshs, with the space-time elements aligned along the (approximate) characteristics. In particular, we address the problem of mass conservation in transferring data between different grids. Numerical examples range from convection-diffusion to incompressible flow in domains with moving parts.