There's really only one way to do it without "hacking" the DNS-323.
Remove the 2x750 disks, install the 2x1.5 disks and create a RAID array - connect one of the 750 disks to a PC running either linux or Windows and the ext2ifs file system driver and copy the data onto the DNS-323.
OK - there is another way - you could backup the data from the DNS-323 and then restore it, and that way you wouldn't have to muck around with the file system driver and installing the disks, but it will double the time you sepnd transferring data and you'll need a place to back it up to.
A word of caution - I've used the ext2ifs driver with XP, I'm told it gives trouble with Vista.