We did the exact same thing a few weeks back in full prod with a lot of writables....
It isn't really hard when you have version 2.15 of Appvolumes.
First things first. Appvolumes checks vcenter to see which datastores are available. That means you will first need to add the new datastores to your vcenter environment and then go to Appvolumes --> Infrastructure --> Storages and do a re-scan.
For the appstacks it is very easy, just create a storage group and add your current datastores and the new datastores and select "Automatically Replicate Appstacks". Appvolumes will copy the appstacks to the new location by itself. Do keep in mind if you press replicate now it will start copying all appstacks using all possible threads of Appvolumes and quite some bandwidth so do this during off hours.
Writables is a different story. Because you have version 2.15 you can move writables from 1 datastore to another datastore. Just select the writable and select move. Keep in mind if you select more than 1 writable you cannot select "move now". It will wait for about 2 to 3 minutes and then start moving the writables (it actually just copies the file changes the database and removes the old writable). Users cannot login with a writable during this time so keep that in mind. If you select more than 10 writables to move it will break the move action up into 10 writables at a time. This is very nice because it won't use all your available threads but still i'd suggest doing this during off hours.
Thanks Ray !.. I greatly appreciate your help. Odd i never received an email from VMware on your reply.
Thanks again.. that makes me feel more comfortable on performing the moves.