By any chance does that include interoperability with the ext2 implementation used on theater projectors? Not a gotcha, if that’s been solved that would make my life tremendously easier.
As far as I know literally all you have to do is mount with -t ext4 (which presumably will just happen by default for ext2 superblocks after this.) As far as I know, it will definitely not automatically enable ext2/ext3-incompatible features, you have to do this manually (though doing so is relatively easy.)
the ext4 driver doesn't (in fact, cannot) upgrade file systems missing features. If you mount an ext2 file system from the 1990s with the ext4 driver, it'll stay at the old feature set, for better and worse.
You can use tune2fs to upgrade file systems, downgrading is not (usually) possible without a full backup and restore.
