I’m pretty sure that as you posted this 4 weeks ago, that you already know the answer, but here’s my experience. Existing drive was a 2 TB (2x 1TB drive) WD My Book Studio Edition II .
Short answer is yes - provided that you’re willing to remove the existing drives , and put in 2 new WD Caviar Green drives (I could only afford 2x 1.5 TB drives), you should have no problem. I haven’t tried this by putting in a single 1.5 TB drive and hoped for a space increase - my guess is, it won’t work.
It is somewhat misleading however.
You DO need to uninstall the WD Drive Manager application first (launch the original installer, then uninstall).
THEN install the latest firmware release for the WD My Book Studio Edition II firmware update
Once you’ve done that and done the precautionary reboot (and follow the instructions, if it’s on FW, make sure you reconnect on USB), it STILL says that it’s a 2TB drive in my info (even though it’s actually 3 TB in my case).
It was only until I installed the latest version of the Drive Manager installer, and reformatted as Raid 1, that it recognised that it was actually 2x1.5TB drives. I am paranoid so wanted Raid 1, but I’m sure that if you also used Drive manager to change to Raid 0, it would also see it as a 3 TB total drive.
So, it does work, but make sure you follow the process, as it can be somewhat misleading along the way (and if you follow the process you probably won’t swear unnecessarily (as I did)).