This patch swaps Mario and Luigi's positions in the game; making Mario the second player and Luigi the first player. No gimmicks whatsoever, the same gameplay of the vanilla game except with different characters.
This hack swaps the character palettes, the in-game text, and even changes the title screen to say "Super Luigi Brothers" unlike the previous mod to swap the Bros on this website.
Enjoy!
Filename: Super Mario Bros (PC10).nes
File SHA-1: D48D2C65FDA380217EAD73FFA6A46CAE62939194
File CRC32: BAD306EF
ROM SHA-1: 5AA663E64AA32602C8AE82C0BBD4EA279EF1A346
ROM CRC32: F21AF99C