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.
The Supreme Court has put its own fingers into the apportionment process, and it has been far readier to find fault with schemes that include multiple member districts or at-large seats, but it hasn't made such schemes automatically beyond the pale. Congress likely went to a mandate for single member districts so as to cut down on redistricting lawsuits, but it would be possible, especially if we got a court that kept out of redistricting issues, to go back to something other than single member districts being used without having to overturn any precedents or amend the constitution.
