Changing Second Profession

Is it possible to change a character's second profession?

Thanks in advance.

Yes. In Prophecies you have to get to Crystal desert and do various quests to allow you to change. In Factions you have to get to Nahpui and talk to profession changer (500g per prof). In Nightfall you have to complete quest Building the Base: Prisoners of War then talk to profession changer in Command Post (500g per prof). After you have a secondary unlocked you can change professions in any town, outpost or guild hall.

Do you lose anything after you change your secondary profession? Will you have all of the skills available after you change or will you have to get them all?

You don't lose anything. All skills you have learned are still available. You can change at will with no penalty

For instance, if I change from a Mesmer to a Monk, will I get all of the Monk skills immediately or will I have to unlock them?

You unlock a few base skills, but the majority have to be learned/bought/capped.

Thank you! =)