You cannot place the entire datastore cluster in maintenance mode, but you place the individual luns comprising of the datastore cluster in maintenance mode. For example, you will have a storage DRS cluster containing luns from both old and new san (provided luns are of similar performance profile), and you will place old lun in maintenance. However, if you want to move VMs to specific luns, then you will have to use storage vmotion, as you already have Storage DRS, it appears VMs can be place on any lun.
Here is a quick powercli script guide for storage vmotion.
http://www.virtu-al.net/2009/06/11/storage-vmotion-the-powercli-way/
http://www.virtu-al.net/2011/09/14/quick-and-simple-storage-vmotion-scripts/