visas are given to company as per their need.they check profile of comp very carefully before granting visa.your sponsor got visa for plumber and they put accountant as plumber.now you say I want to be accountant and you may be have qualification but your company have qualification to hire accountant?may be they have ac before. so what you have to do,is make agreement between you and company as accountant then you (or mandoob)have to go to regester it at labour dept with letter to change you as ac.if they approve then immigration will change it.this is normal chanel.