Not being able to go back is perfectly acceptable for a new filesystem. Arguably less so for the same filesystem.
I would except a kernel with support for ext3 to be able to mount a ext3 filesystems. Everything else is, imho, lunacy.
There's no reason to expect though, that an ext3-driver should be able to mount a ext4-filesystem. If the oposite works as a migration-path it's a big benefit, but that's it.
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