Not to be funny, but are you sure you joined the Arcanist guild and not the Alchemist guild ?

Class switching is basic, once you equip the weapon item slot for that class you become that class, when you joined the Arcanist class you should of gained a Grimoire and the spell Ruin I and probably lost the ability to wear most of your level 20 Conjurers clothes... to switch back to Conjurer you simply equip the rod/wand of your Conjurer (or use the gear set system and create a gear set for each class, note you can accidentaly delete items one class is using and if its the weapon in the gear set it will break the gear set).