From the readme: This is a rudimentary patch that swaps Mario and Luigi ingame with accompanying modifications to game text, screens, and HUDs. No changes have been made to the game’s mechanics. …

• Luigi will replace Mario as the first player.
• Mario will replace Luigi as the second player, only being playable in 2-player mode.
• All game text referencing Mario and Luigi has been swapped to correspond to these changes.

This patch functions on all regions and editions of the game, including Duck Hunt versions.

• Database match: Super Mario Bros. (World)
• Database: No-Intro: Nintendo Entertainment System (v. 20180803-121122)
• File SHA-1: EA343F4E445A9050D4B4FBAC2C77D0693B1D0922
• File CRC32: 3337EC46
• ROM SHA-1: FACEE9C577A5262DBE33AC4930BB0B58C8C037F7
• ROM CRC32: D445F698
• Database match: Super Mario Bros. (Europe) (Rev A)
• Database: No-Intro: Nintendo Entertainment System (v. 20180803-121122)
• File SHA-1: AB30029EFEC6CCFC5D65DFDA7FBC6E6489A80805
• File CRC32: 7D5FAA58
• ROM SHA-1: 0C4992FC08D2278697339D3B48066E7B5F943598
• ROM CRC32: 9A2DB086
• Database match: Super Mario Bros. + Duck Hunt (USA)
• Database: No-Intro: Nintendo Entertainment System (v. 20180803-121122)
• File SHA-1: 663DDEF71CC808C5382A3426EF6D03B5499A51C8
• File CRC32: 666EE28E
• ROM SHA-1: 36E695BA6CACC8A5098FB67D9C1F8E6828D6C2C6
• ROM CRC32: D26EFD78
• Database match: Super Mario Bros. + Duck Hunt (Europe)
• Database: No-Intro: Nintendo Entertainment System (v. 20180803-121122)
• File SHA-1: D88140962DDAD5B852928E2E3EE88F295456D250
• File CRC32: 25B4AE21
• ROM SHA-1: 455773982E332BFC30E5CF33CDF5475F2B5E0490
• ROM CRC32: 91B4B1D7