Are there many differences between the SNES original and this GBA game?

Varies from game to game.

Specficly in Yoshi's Island's case:

List of changes

• The Pause Menu has changed. Instead of one menu handling all functions, there are now two. One allows the player to put their Game Boy Advance into sleep mode, or return to the overworld map. The other, which is a redesigned version of the original's pause screen, allows the use of items, and displays the player's current score for the level.
• The Yoshi's Island Mini battle code was changed (because the Game Boy Advance didn't have  and  buttons) to , , , ,  (while holding the  button). The 2-Player option was also removed due to a link cable battle being only featured in the Mario Bros. remake.
• The Countdown timer ticks down each second now, instead of nearly twice per second as in the original game.

Level design changes

The World Map of Yoshi's Island when beginning the game.

• A new room has been added to the level 1-8 (Salvo the Slime's Castle). It is between what were the second and third rooms in the original game. It has one Shy Guy-spawning pipe.
• The original version had 21 red coins in More Monkey Madness (Extra 3), but the remake has the usual 20. There was a red coin under a Tap-Tap on a ground pound stake. This has been removed from the remake.
• An extra Middle Ring has been added to 4-4 (Marching Mide's Fort).
• The final room before the boss in 4-8 (Hookbill Koopa's Castle) has been redesigned.
• All of the extra levels in the original game were modified for this version, most noticeably Kamek's Revenge.
• Six entirely new levels were added to the remake, called Secret Levels. These are to the left of the Extra Levels on the map screen, and are unlocked by defeating Baby Bowser on level 6-8. Consequently, it is no longer necessary to have a perfect score in a particular World, to unlock that World's extra level, but if the player gets all 100 points on each level, they will go to a secret level.
• The final level of the game had its name changed from "Castles - Masterpiece Set" to "Ultimate Castle Challenge".

Graphical changes

• As always, the palette is lightened to combat the original Game Boy Advance's lack of a backlight.
• In the original, the Yoshi who completed the x-2 levels was a very dark shade of pink. However, they appeared red on the map. Similarly, the Yoshi who completed the x-7 levels was red, but appeared hot pink on the map. In the remake, the x-2 Yoshi is always light pink, and the x-7 Yoshi is always red.
• All Yoshis had red shoes in the original. In the remake, they have different colored shoes, as in Yoshi's Story.
• When Yoshi touches a Fuzzy in the original game, the background smears erratically and fades between random colors. The ground also ripples. In the remake, the background doesn't smear or tint. However, it does fade between white and its normal colors. The ground ripple is also less fluid in the remake. This probably happens because the Game Boy Advance is not able to handle the Super FX chip-powered graphic effects as well as the original.
  • Probably related to the above, Red Coins were given away in the original; while yellow coins followed the rippling of the ground, Red Coins did not. In the remake, all coins ripple with the ground.
• In underground levels with "sparkling" ground, mushrooms and other foreground decorations sometimes have their palettes changed. This is done to fix a glitch where some black objects would also sparkle.
• Sealed doors have different graphics. The boards that cover them aren't visibly bisected, as in the original.
• Kamek's magic has a different visual effect. The colored cloud is absent, while the cycling sparkles remain.
• The overworld map was redesigned. The path the Yoshis follow to their goal is now in a straight line (due the GBA screen limitation), and spots have been added to the map for each world's two hidden levels. Additionally, a "world select" screen was added, and if the player has not yet completed World 6-8, Baby Luigi is shown on World 6's spot on the world select map. He is seen in a bubble, crying, much like Mario when he is knocked off Yoshi's back.

Sound changes

• The game's soundtrack was downgraded to match the Game Boy Advance's sound capabilities.
• The full length opening of the World x-8 boss music plays for all big bosses, rather than for just the Navel Piranha and Rapheal the Raven.
• In World 6-8, when being chased by Tap-Tap the Golden, the "path to the boss" music is played, rather than the World x-8 boss music.
• Several sounds in the original were replaced by sound clips similar to the ones heard in Yoshi's Story most notably the Shy Guys' and Yoshis' voices.
• When Yoshi jumps, flutter jumps, and crouches or throws an egg, he will make sounds instead of staying silent.

Textual changes

• Much of the wording has changed throughout the game's dialog. Notable examples include:
  • In the opening cinematic, the story in the SNES version starts "A long, long time ago... This is a story about baby Mario and Yoshi..." The GBA version starts out "This is a story about Baby Mario and Yoshi... A story that took place a long, long time ago.
  • At the world 3-8 boss, when Kamek appears, he says in the SNES version, "Give it up, Yoshi, you cutie without a navel! Ooopp- forget it..." whereas in the GBA version, he says "Give up, Yoshi, you egg-throwing maniac! Ooops... Forget that!"
  • Finally, when entering Bowser's room, Kamek says "YOU! are n-n-not welcome HERE!!! Yoshi, please hand OVER THE BABY!" in the original, and "Welcome!!! Yoshi, if you would be so kind, please HAND OVER THE BABY!" in the remake.
• The GBA version's credits are shorter than the original, and list different staff members. As this is a remake, the credits list "Original YI Staff" under "Special Thanks"; it should be noted, however, that very few of the SNES version's staff members actually contributed to the remake. Notably, as Hiroshi Yamauchi resigned from the presidency of Nintendo a few months prior to the GBA version's release, the remake's executive producer is his successor, Satoru Iwata. Not even Shigeru Miyamoto is credited in the remake; Takashi Tezuka, one of the original game's directors, assumed producer duties in his stead.

Six new levels...I like the sound of that.

Week 18 In the Virtual Console

(NES) Mach Rider (NA)

(NES) Castlevania II: Simon's Quest (PAL)

(GBA) The Legend of Zelda: The Minish Cap (Japan)

(GBA) Kirby: Nightmare in Dreamland (Japan)

(GBA) Mario Tennis: Power Tour (Japan)

(GBA) Wario Land 4 (Japan)

(T-16/PC Engine) R-Type (Japan)

(T-16/PC Engine) China Warrior (Japan)

(GBA) Mario & Luigi: Super Star Saga (Japan)

(GBA) Metroid: Fusion (Japan)

Confirmed for Next Week

Japan: Wagan Land & Pop'n TwinBee

Newly Listed Games

PAL: Pop'n TwinBee

I need dual citizenship.

Yeah, Japan got a lot this week.

Week 19 In the Virtual Console

(GBA) Wario Land 4 (NA)

(NES) Volleyball (PAL)

(SNES) Pop'n TwinBee (Japan)

(NES) Wagan Land (Japan)

Confirmed for Next Week:

Japan: Fire Emblem: The Sword of Flame, Megaman 6, and Princess Tomato in Salad Land

Newly Listed Games:

NA: Megman: Battle Network 4 (Blue & White)

Week 20 In the Virtual Console

(NES) Adventures of Lolo (NA)

(GBA) Mr. Driller 2 (PAL)

(GBA) Fire Emblem (Japan)

(NES) Megaman 6 (Japan)

(NES) Princess Tomato in Salad Land (Japan)

ohhh I remember the adventure of Lolo

Same, I think there was even some sequels.

On another note, Fire Emblem GBA?

DO WANT!!!

*Is greedy and and wants Sword of Seals as well*