Quote Originally Posted by Unholychinchilla View Post
That's because creation was cycled out mid-expansion. At the start of a new expansion the previous expansions tomes get rolled into poetics. This is exactly what happen at the beginning of Stormblood, where the last two tomestone types (Lore and Scripture) could be traded in for poetics. The beginning of Heavensward was a bit different as it was the first expansion and poetics was originally the final capped tomestone of A Realm Reborn so it didn't need to be traded in.

Don't think of it as trading in Genesis being traded down to poetics, it's more that poetics is having it's level range brought up to include Genesis gear and replacing Genesis.
I wanted to thank you, by the way. I wasn't joking or making things up when I said that it's a tough subject to simply google.