Calculations of magnetisation reversal via a perpendicular exchange spring in soft-hard bilayers are presented using the example of an FePt∕FeRh bilayer which has recently been proposed as an alternative medium for heat-assisted magnetic recording applications. We show that the standard three-dimensional micromagnetic model fails to describe the coercivity reduction due to the fact that at low interface exchange value, the domain wall penetration into the hard layer is not correctly described within the continuous approach. A multiscale model based on atomic level simulations near the interface and a micromagnetic approach far from it is proposed. This leads to improved calculations of the exchange spring at the interface, allowing a detailed study of the magnetisation reversal dynamics. As a result, the coercivity is found to saturate at an interfacial exchange value of 4% of the bulk value, the practical applications of which are discussed. In addition, the granular structure in an FePt medium with different intergrain exchange parameters is considered.