Upgrading from airline First Officer (copilot) to Captain involves more than simply moving from the right seat to the left. If a new type rating is required, there will be ground school and simulator training, and the ubiquitous check ride.
Simulator training may consist of traditional Appendix H Training to ATP Practical Test Standards and the newer Advanced Qualification Program, and will be conducted in a Level C or Level D simulator.
After training is complete, the new Captain must complete Operating Experience (OE) - formerly called Initial Operating Experience (IOE) in accordance with FAR 121.434, which consists of 25 hours of supervised inflight training on regular revenue flights with a Line Check Airman in the right seat. At the completion of OE, if it the pilot's initial Captain certification, an FAA Aviation Safety Inspector will ride along on one leg of the OE to observe the PIC's performance during the latter stages of OE.