Sindyth One-Hit no longer exists. Gossamer ranks are used instead. If you've trained any Sindyth, you'll have the experience from those ranks to re-train into something else when you first log in. If you're currently training Sindyth, you'll need to pick another trainer.
You can only study one beast at a time. If you want to switch to a different one, you'll have to abandon your studies of the current one, losing any progress you've made toward learning more about it. People who have already begun studying more than one creature before v412 will be able to switch among all of those, but they won't be able to start anything new until they've either finished or abandoned all their current studies.
Gossamer training now also gives some Duvin Beastlore, much the same way Evus gives some Atkus. (Not the same numbers, of course.) If you've already trained with the gossamer, you've been given free ranks of Beastlore to match the amount it now gives. If you're currently training with the gossamer, it has been updated to the new mix.
Ann