Most likely you'll be able to convert Myth>Sold tomes when the time comes. Myth Armor is currently available as Hunt/Allied Seal exchanges, so I actually doubt they'll junk them into the Dungeon RNG... Darklight which was the old Philosophy Tome items weren't ever put in anywhere aside from dungeons, but Myth Armor has been in Hunt from (likely) the start of The Hunt.

Thavarian Mist'll probably be converted into a Sold or GCS or even an AS item(my bet's on Sold though). I'm personally a little more interested what they'll do with the i90 crafting items currently for 500 Myth will be shoveled into.