> old kernels will not be able to use the new format (even in read-only mode)
> new kernels will be able to use both the old and new format
> once a filesystem is converted, there will be no way to 'unconvert' it.
There is no "convert" or "unconvert" operation - the format is selected at mkfs time and it is fixed for the life of the filesystem. That's an explicit design decision (as I've previously described)....
> This sounds reasonable (although may still be reason enough to call it
> XFS2 even if it's the same codebase supporting both on-disk formats)
A name change would only cause confusion. XFS has a history of mkfs-only selectable format changes over time and this change is being handled in exactly the same manner as all the previous ones that have been made. If XFS was renamed every time the on-disk format changed then it would be XFS 17 by now. ;)