In actual fact that expansion was a ghost. Lots of whirring and it truly went through the motions, but after two rebuilds and about 14 hours… back to square 1. I opened a support case - and then followed the procedure in Is deleting a volume possible? - #4 by RogerJP
Briefly to answer the original post, the drives are compatible, but look for the “expand” tick early on - before the very first 16TB insertion.
