Werewolf/Vampire question

Hi I'm playing skyrim for the first time, I'm a werewolf but ( I think as part of the dawn guard DLC) some vampire lord offered me to become a vampire. Can I become a werewolf again and can you generally always switch between being a werewolf or vampire? Do I lose my progress in the werewolf skill tree if I become a vampire and have to start from zero when I become a werewolf again? Thank you very much in advance ! :)