Even if you are type rated the in the airplane, there is a lot more to upgrading than learning how to fly the airplane from a different seat. You'll find that most of the real-life challenges you face as Captain have nothing to do with engine failure on takeoff!
At many airlines, when it took more than 10 years to make Captain, copilots would have a lot of exposure to good and bad Captains, and would have the opportunity to see countless airborne decisions and evaluate their results. With rapid advancement now days, it's possible copilots will not have the extensive mentoring that existed previously.
At most airlines there is some form of New Captain training to give the prospective aircraft commander training and instruction on a variety of operational topics, such as Leadership, Crewmember Mentoring, Crew Resource Management (CRM), Inflight Medical Issues, Decision-Making, Management, Fatigue-Risk Management, Stress, Aviation Law, Company Procedures and Performance.