How full is your NAS? If you're not that full and your data could fit on one non-RAID hard drive, you can add a 4th drive as a second disk group, copy folders manually in Diskstation to the 4th drive, then pull the other three, create a new 3-disk disk group there, copy back once that's built, then wipe the 4th and add that. That's convoluted but for me was the easiest and most reliable way. With the RAID max_speed tweaks, it was all done in about 72 hours end to end.