have purchased shiny new 8tb drives to replace the 4tb drives in my qnap nas (I need ~3tb to store my rips). it really does seem to have been as simple as finding the "imma replace this drive" button in the web ui, then swapping out one of the drives. it's at 0.7% rebuilt now...we'll see what it looks like in the morning.
damn...not bad. already up to 70% restored. count me as a happy qnap customer so far, y'all.