played and completed.
here are a few changes that Nintendo made for the GBA port. This list is not accurate. Just to give you an idea of what to expect!
graphic/sound changes
-New intro sequence. bowser's ship lands in Grass Land and a Koopa Kid takes over the castle. The first time you visit a castle, an intro scene showing a Koopa Kid stealing the wand and turning the king into an animal.
-General graphics/sounds are a little upgraded. If you played hard the SNES version you will notice the changes. For example coins and cannonballs are a lil bigger because of the reduced screen size.
-Voices. Only Mario and Luigi have speech (the usual Let's Go!; Mamma Mia; Just What I needed...) except for an appearance of Peach crying MAAARIOOOOO! after you beat the first two tanks in World 8. Almost forgetting, Boos do screech! (cool sound effect!)
-When warp-whistling, the SMB2/SMA1 "warping screen" effect is used. Then appears a message that (I guess) says "welcome to world 9"
-If you warp to world 8 you receive Bowser's message as u would if you beat world 7. If I recall correctly this didn't happen in the SNES version.
-Introduction to mini-games is way improved! It shows you and Toad at a table. Toad says stuff and then you enter a door where presumably the game machine is located.
-picture-completing minigame sound change: if you match the first 2 pieces of a picture, a drum roll effect will play.
Mario says "BRAVO!" if you win.
GAMEPLAY CHANGES
-the game is a LITTLE easied up, in accordance with Nintendo "No-Gameover" policy. When I completed the game 100% I had about 60 lives left and a 1.7M score.
-Beating Bowser unlocks the "world select" screen. Yes, you get to pick what world to play in! If you beat all levels (90/90)then you can freely play the levels you want.
-2 players option: you have the option to play as "mario only" or "mario and luigi". The second choice is made to play with a fellow player, taking turns at the console. You can transfer lives between the players if you wish.
-You can pick a box in Toad's house even if Toad is still talking. Also you can interrupt Toad when entering minigames.
Just you must wait for the speech to be completed while in the king's presence(respect the boss!)
-you get to play the card-matching more than once if you get 80000 points more than once (if you get for example from 79950 to 160000 in one level you get to play the game twice)
-super picture-completing game! It appears as a hearts symbol on the map (I found it only in World 2). It is the same of the spades-picture-completing but instead of Starman there is Raccoon Leaf as top life givers. Complete the Leaf for SEVEN lives!
-Pressing L on the map screen shows your item reserve. Pressing R a Lakitu appears and drags in a cloudy stage. Then he asks you for the e-reader. I guess it's for unlocking the famous hidden features.
-you can reach "World e+" from the select screen but it's useless if you don't have an e-reader (and the appropriate cards). The map screen contains: a space that allows you to switch between Mario and Luigi; a Lakitu spot that teleports you to the cloudy stage; some mushroom houses, all locked; and also a castle space. When you enter it you'll find it empty. on the upper-left corner the 1F message tells you that you are on first floor... looks like you can add stuff to that castle with e-cards.... maybe stairs!
Ace coins and other weird stuff are not unlockable by playing the basic game alone.
That's what I found for now. now get out and play!