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This just means you can take the feat at any level (1, 5, 9, 13, 17) but once you do you need to keep it. A player might choose another level 1 ancestry feat and then choose to grab Mighty Dragonet at level 5, for instance.
Most ancestry feats can be retrained. Ones that offer a physical alteration often come with language about being unable to retrain them or being forced to take them only at level 1 and then not being able to retrain them. This includes all Lineage feats in Versatile Heritages or feats like Bestial Manifestation
I'm aware of how the retrain restriction applies on physical change feats, they're usually my favorite feats, it was the first half that was tripping me up because I felt like the phrasing was weird. I still do honestly, but it makes more sense now.