Currently, if you already have a regular index for a given field, there is no way to make it sparse without first deleting the regular index.
But this is painful when upgrading from 1.6.x, because during the time that the sparse index is being created all your queries that rely on that index become table scans.

Would be better if there was a way to replace an existing index with a sparse one with no time during which neither index is usable.