I just found this thread
which seems to ask a rather similar question and apparently the answer is no - it does appear that I indeed need to be rooted if I want to downgrade. It seems that if I use the above OTA method without root there will be problems with the bootloader as far as I understand?
I've flashed all my recent Nokia phones without bricking them, but I'm not so sure I want to take the chance on a tablet that is just two months old in case something goes wrong. I think for now I'm stuck with Jelly Bean (until I'm willing to root the device) or wait for some kind of update that gives the user an option to customize the UI or install a tablet UI without being rooted, but I'm not holding my breath haha.