two ways..
the easy way is to copy/migrate the indexes (or entire nsr) to a fresh partition with plenty of space and then copy the data using rsync or a tar pipe (taking care to preserve permissions) and then mount the new partiton in the appropriate place (e.g. /nsr if the whole /nsr directory is migrated to the new partition).
The hard way is to move the individual indexes and then point the index location (set on each client) to the new location.
HTH
_m