  Distributed storage systems must store large amounts of data over long
periods of time. To avoid data loss due to device failures, an $[n,k]$ erasure
code is used to encode $k$ data symbols into a codeword of $n$ symbols that are
stored across different devices. However, device failure rates change
throughout the life of the data, and tuning $n$ and $k$ according to these
changes has been shown to save significant storage space. Code conversion is
the process of converting multiple codewords of an initial $[n^I,k^I]$ code
into codewords of a final $[n^F,k^F]$ code that decode to the same set of data
symbols. In this paper, we study conversion bandwidth, defined as the total
amount of data transferred between nodes during conversion. In particular, we
consider the case where the initial and final codes are MDS and a single
initial codeword is split into several final codewords ($k^I=\lambda^F k^F$ for
integer $\lambda^F \geq 2$), called the split regime. We derive lower bounds on
the conversion bandwidth in the split regime and propose constructions that
significantly reduce conversion bandwidth and are optimal for certain
parameters.
