Will ZFS require a complete new install, or will "upgrade in place" be possible?
Intuition says, it will require reformatting the drive, i.e. completely new install, then again, I've seen in other contexts some installer magic where one might expect the requirement for a fresh install, too, and file systems were converted on the fly.
Main reason I'm curious is that I have a remote system...
Yes reinstall is required to use zfs.
yeah anytime you change the file structure your going to have to reinstall stuff.