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.
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)
what you initially said sounded more like
old kernels will not be able to use the new format
new kernels will not be able to use the old format
when you upgrade the kernel you will be forced to convert the filesystem, and there is no way to unconvert it.
This is not acceptable and is the reason people were concerned.
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