The weapon is not locked to the class. The Job is locked to the class - unless some of you think we can transform Crab Bows into harps.

Although not confirmed, I greatly suspect there will be job specific weapons and by equipping said weapons, similar to how classes work, one will change to that job - provided they have unlocked it.

The difference with jobs will be that they will only use skills from the class they are linked to and their own job specific skills, and will not have cross-class skills. Stat wise it is also expected they will be highly specialized to their purpose. Jobs also do not have their own level, but share experience and level value with the class they are linked to.