So I've read various pages and see that the recommended method to replace a data drive with a larger one is to simply unassign the old one, assign the new one, and let it rebuild onto it.  My question, out of curiosity, is how the filesystem is grown (since there don't seem to be any instructions for it and people seem to say that it "just works").  I would assume that the kernel-level unraid md driver only literally rebuilds the disk image.  Does emhttpd throw some magic on top of that that edits the partition table and grows the fs after the rebuild is done or something?  Just want to understand exactly what I'm doing before I proceed (I'm a nerd!)

Sort of. I think the way it's implemented is that array devices are expanded as part of the mount process. I'm pretty sure I've seen the messages in the syslog about that, so maybe take a look and see what you see?

Thanks!  I see that it does xfs_growfs on every disk it mounts   And runs sgdisk to create a new partition table on some subset of disks, seems most likely newly-added ones.