To earn money for flying aeroplanes, you have to - by law - hold a commercial pilot's licence. A basic commercial pilot's licence (CPL) entitles you to fly as pilot in command (PIC) of small aircraft. An Airline Transport Pilot's Licence (ATPL) entitles you to fly in command of large aircraft.
It doesn't matter what your aircraft carries, human passengers, animals or boxes, the licencing requirements for the pilots are the same. A pilot flying cargo today is licenced to fly passengers tomorrow with no extra training or qualifications
Not quite...
Depending on the country rules, a CPL or ATPL may be required to fly passengers OR cargo for hire in ANY airplane. In the US, for example, the PIC in a Part 121 operation must have an ATPL, but the SIC may have only a CPL+IR in domestic and international 2-pilot operations. For Part 135 operations, the ATPL is required for passenger operations in airplanes with 10 or more seats.
Whether a pilot can currently fly cargo or passengers depends on the certificate and training program of the operator.
In general, Part 121 operators (airlines) have requirements that are more stringent than the FAA's. Most require ATPLs for all pilots, and their training programs encompass all intended operations, so all pilots are qualified to fly all operations in their assigned airplane type.