Everybody's been asking so here is a quick guide.
At level 35, 37, and 39 you can accept your level 40 class transfer quests. When these quests are picked up you will receive dimensional diamonds, these diamonds can be exchanged at a GK(Gatekeeper) for teleportation scrolls to main cities. To get the maximum profit you want to exchange all your diamonds for Scroll of Escape: Talking Island. Sell these to a NPC shop like the grocery store merchant for 17.5k each. Depending on your class you can make anywhere from 700k-1kk.
Here is a quick example of a level 39 Elven Knight.
Although the Elven Knight wants to transfer to a Sword Singer he will still pick up all the three beginning class transfer quests for Temple Knight; Trial of Calling(35), Testimony of Life(37), and Test of the Healer(39),He will also go accept all the class transfer quests for Sword Singer;Trial of the Challenger(35), Testimony of Life(37), Test of the Duelist(39). In this scenario Testimony of Life is used for both class transfers so you will not receive Dimensional Diamonds twice for picking up this quest.
YOU CAN STILL DO THIS EVEN IF YOU DECIDE TO PAY FOR YOU CLASS TRANSFER WHICH IS $5 ON https://l2reborn.com/donations/