Yes it is possible with the new code. However it isn't quite released yet (Though you can pull it from my git tree). You will need mdadm-3.1 and linux-2.6.31.
If you just want to change the chunk size, the new mdadm will do that for you
mdadm --grow /dev/mdX --chunk-size=128
If you want to reduce the number of devices in an array you can. You will have to
shrink the filesystem first, then shrink the array with
mdadm --grow --array-size
and then change the number of devices.