Real-time systems have an increasing demand for predictable performance. Only recently novel models and analyses were proposed that make the performance benefits of runtime-reconfigurable architectures accessible for optimized worst-case execution time (WCET) guarantees. However, the implicit assumption in these works is that the process of reconfiguration itself complies with execution time guarantees. The realization of a reconfiguration controller that fulfills these assumptions and that is amenable to WCET guarantees is so far unavailable. In this letter, we detail the challenges of runtime reconfiguration in real-time systems and show that conflicts while accessing a shared main memory during reconfiguration can lead to a slowdown of more than 21× in reconfiguration bandwidth. We present concepts that enable runtime reconfiguration under WCET guarantees and release our implementation of these concepts as open source.