This article is devoted to the run-time redistribution of arrays that are distributed in a block-cyclic fashion over a multidimensional processor grid. While previous studies have concentrated on efficiently generating the communication messages to be exchanged by the processors involved in the redistribution, we focus on the {\em scheduling} of those messages: how to organize the message exchanges into «structured» communication steps that minimize contention. We build upon results of Walker and Otto, who solved a particular instance of the problem, and we derive an optimal scheduling for the most general case, namely, moving from a {\tt CYCLIC(r)} distribution on a $P$-processor grid to a {\tt CYCLIC(s)} distribution on a $Q$-processor grid, for {\em arbitrary} values of the redistribution parameters $P$, $Q$, $r$, and $s$.