There are 2 methods you can try:
1) disk to disk cloning
2) NAS to local drive, local drive to other NAS copying.
Number 2 first. I ran into this situation recently where I was migrating a 500GB folder from one NAS to another. Scheduler just kept messing things up as it wouldn't properly do a folder from the root to the root of the other NAS as it wants it to be to a folder, and when it did do it I had missing data and or security was changed on some files making them inaccessible (plus if it stops while copying it can make the folder undeletable). Best way I did it was to use TeraCopy, copied my data to my local machine, then to the other NAS. Unless you don't care about this, DO NOT use Windows copy/paste as their creation/modified dates will change. Also, doing it this way is faster than NAS to NAS copying since you use your full networks bandwidth in one direction at a time rather than trying to go full tilt in 2 (doing so can increase your chance of packet loss or data corruption).
As for disk to disk cloning, this might be your best bet. Hookup the drives to a PC, and boot up using something like Norton Ghost (10 or higher), or Acronis True Image Home Edition 2010 or 2011. Both straight copy data AS IS (copying using Linux or Windows copy/paste can mess things up security wise, and data wise), and you can manually resize the partitions, or let the application do it for you (just be sure to leave the smaller partitions as is, and resize ONLY the data partition, and for Acronis be sure to uncheck the option for deleting the old partitions). I've used this method countless times for clients as well as myself when I migrated from 250GB to 500GB drives, and then to my current 1TB drives. It can take time, but it works like a charm.
The only issue I can see is that you are running RAID. Since I've never done the disk to disk cloning on a RAID setup I can't say what steps you need to do in order for it to work properly. You may need to setup the RAID first on the new NAS, clone BOTH drives, then restart the box with both in, or if you should clone 1 drive, insert it first, shut down, insert the other drive so it shows up as Volume_2, then RAID the drive (however I don't know if doing so will maintain the data on Volume_1, or erase it, and start a fresh RAID). Someone who knows more about running RAID on these might be better able to address that part of it.