No unfortunately.

When you hit level 30 on a particular class you open a questline that allows you to open up a "Job". These jobs are things like paladin and monk. The requirements to take on these quests are as you stated above.

Once your in a job then you are that job. You can however equip a select amount of abilites from other classes that are set for it. For example Paladin is able to use all the skills from gladiator, and some select skills from marauder and conjurer. You don't however get to use all skills acquired over the course of the game. The classes that each job can use is limited to 2 in addition to the all the skills that the class that it originally came from.

In exchange for losing the variety that you would have with a class you get more powerful abilities that center around that Job's role.