Not only does it not require an amendment, Congress could mandate a proportional system, just as it has mandated the use of single member districts and no at-Large seats.
Congress has explicit authority with regard to the time, place, and manner of electing members of Congress. They have required election from single member districts. The USSC has required a extreme level of population equality.
Congress only has the authority to set the date that presidential electors are chosen and when they cast their votes. An exception is for the electors for the District of Columbia where they act as the equivalent of a state legislature.
