  An arborescence in a digraph is an acyclic arc subset in which every vertex
execpt a root has exactly one incoming arc. In this paper, we reveal the
reconfigurability of the union of $k$ arborescences for fixed $k$ in the
following sense: for any pair of arc subsets that can be partitioned into $k$
arborescences, one can be transformed into the other by exchanging arcs one by
one so that every intermediate arc subset can also be partitioned into $k$
arborescences. This generalizes the result by Ito et al. (2023), who showed the
case with $k=1$. Since the union of $k$ arborescences can be represented as a
common matroid basis of two matroids, our result gives a new non-trivial
example of matroid pairs for which two common bases are always reconfigurable
to each other.
