BTRFS can upgrade from ext4 by slotting itself into free space and marking the old metadata as a special snapshot. The same thing should work on other filesystems with few new challenges.
Support for that has been dropped within Btrfs AFAIK. Also, "upgrading a filesystem in place" is more of a lottery than a feature that people should actually use. So many bad things can happen.
Well I did it a month ago, on a reasonably-updated system, at least.
The main point is that upgrading "in-place" does not require any of the metadata to be in the same place between old and new filesystem. With a mildly flexible destination filesystem, the old filesystem can stay there until you're completely sure the conversion is a success. You end up with two read-only filesystems sharing a partition, and you choose which one to go forward with. Cancelling at any point is trivial, even if the conversion process crashes.
