Often real-time systems can run in different modes depending on the external environment or their internal state. Each operational mode is characterized by a set of tasks with different computational demand, resource requirements, and resource availability. When resource reservation is used to achieve temporal isolation among applications, the reservation parameters may need to change from mode to mode. Hence, an additional guarantee is required to ensure feasibility not only of the applications, but also of the reservations. This paper presents a schedulability analysis to predict the timing behavior of a multi-moded resource reservation, whose parameters may change due to a mode transition. Resource provisioning is analyzed in all the operational modes and also during mode-changes in order to guarantee a minimum amount of resources and derive a feasibility condition for realtime applications and reservations. Theoretical results are also illustrated with examples and test cases.