The aim of this paper is to discuss the modelling and optimisation of condition-based maintenance policies for systems submitted to usage profile changes. The considered system undergoes a monotone deterioration (gamma process) and is impacted by the usage conditions (covariates) via the proportional hazards model. Four different policies are proposed and the optimal maintenance parameters minimising the long-run average maintenance cost are derived. The different maintenance policies are numerically compared using Monte Carlo simulations.