well right now im a ranger/mesmer and i wanna change mesmer to monk is that possible?

As long as you have ascended in your primary campaign (Where the character was made) you can change at the Isle of Balthazar.

In the three campaigns themselves you can change at:

Prop : Killing your doopleganger is Ascension after that hea d to Isle of Balthazar or do the quests found in the Crystal Desert.
Factions : Senji's Corner (After the Naphui quarter mission which is the Factions version of ascension).
Nightfall : Can't for the life of me remember but it's after you kill all the bugs in the Sunspear Sanctuary.