The wiki states:
In Dawnguard, if the Dragonborn is cured of Lycanthropy or becomes a vampire lord, they can talk to Aela and she can make them a Werewolf again, canceling out vampirism. To become a Vampire Lord again, they need to ask Serana or Harkon (If sided with the Vampires). If sided with the Dawnguard, you have to complete the Dawnguard quest line.
So basically, if you have sided with the vampires, you can hop back and forth between the two forms as many times as you like.
The only catch is that if you have sided with the Dawnguard, and have completed the quest line, you cannot be offered the gift of becoming a Vampire Lord anymore. (This can be glitched though).
To cure each curse, however, there are conventional methods for both.
As part of the main Companion Questline, you will be required to find and kill the Glenmoril Witches, and collect their heads. There are 5 in total. You require at least 2 to complete the quest, one for Kodlak, and one for yourself. The three others can also be offered to other members of the Companions, but can also be kept for yourself, should you choose to re-contract Lycanthropy.
Vampirism (Vampire Lord)
There is also a conventional method to cure this form as well, though for some reason, it isn't quite as majestic or obvious as it is for the Werewolves. Quite simply, you just need to talk to the bartender in Morthal, and ask if they have heard any rumours. You will eventually be offered a quest to find a man called Falion, who will be able to cure the curse of Vampirism for you.