Welcome to the Community,
There's no option to convert the block size other than reformatting the volume. However, since you are posting in the ESXi5 forum, I assume you are running ESXi5 which allows to upgrade the datastore to VMFS5, which allows file sizes up to ~2TB with a unified 1MB block size. Before you upgrade the file system, ensure there's no need anymore to access it by an older host!
André