visa status change/ sponsor change

I was issued a trainee visa for a Local firm and it had not yet been used. And  I was hired by another firm just recently for a permanent job, the new firm will be applying an employment visa for me with the HK immigration. Now here is my question:

 1. Do I need to cancel the trainee visa before my new employment visa be lodge in?

 2. If yes, can I apply for the new employment visa immediatelly after its cancellation?

 3. What are the documents and information needed and how long could it take before the new visa can be issued. the firm is a newly establish firm with head office in dubai.