As per usual, Super Mario Bros. 3 went through a few changes to become part of the Mario Advance series. Due to the fact that a Mario Bros. two-player mode is always included with Mario Advance titles, the Mario Bros. mode was removed from SMB3's two-player mode. In its place is an "extra lives swap" à la Super Mario World, but the card-swapping feature from the original SMB3's Mario Bros. mode is nowhere to be found. Because the B button is used to start the extra lives swap, opening and closing your item inventory was assigned to the L button. The R button accesses special e-Reader options.
Minor changes include slight modifications to levels (i.e. wider platforms and extra coins) which make the game a tad easier, but in most cases the changes are so small that you would've had to have played the original game many times to notice. Another particularly neat addition is a short intro scene showing Larry Koopa storm the castle in World 1...
The screen dimensions of the GBA are a bit smaller than the NES (240×160 vs. 256×224), of course, so data from the original bulky status bar was shrunk to one line and moved to the top of the screen. The power meter remains on the bottom of the screen, out of the way. The three squares showing cards you've collected from Roulette Blocks are in the bottom-right of the screen, but during a level they'll only appear briefly when Mario enters a level, and they'll pop back up when Mario is at the end of a level near a Roulette Block.
Swiping special cards (sold separately) into an e-Reader connected to the GBA unlocks new levels, level "demo movies," and instant power-ups.