What happens if a doctor wants to switch specialties (ie. internal med to cardiology) do they have to go through a whole new residency?

Sort of: There are two types of training - residency and fellowship. They're really the same except fellowship is after initial residency training. Cardiology is a specialty after internal medicine (IM). Essentially, any change of specialty such as IM to general surgery or IM to cardiology requires training for that specialty (anywhere from 2-5 years) to be certified and practice the new specialty.

Fellowship : Cardiology is a subspecialty of Internal Medicine, requiring a three year residency, followed by a 3-5 year cardiology fellowship, depending on whether extra time is needed to learn special procedures, such as stenting or electrophysiology testing and treatment. To switch to a new internal medicine subspecialty (like gastroenterology, rheumatology, etc., a new fellowship would be required.