Apparently, at the beginning of production, they actually made new sprites for the characters! Personally, I prefer them much more over the ones they used in the final, which are just Mario Kart 64 sprites with ugly colour reduction. These ones here look rather cute, with the characters done in a Super-Deformed style. Why would they change them? Unless people bitched they were copying Konami Krazy Racer's style. Still a pretty silly excuse.
So now onto the images. We'll start with the first one. Mario, Wario, and Bowser are the only visible racers, the background is from Mario Circuit, and the tile set seems similar to the same course, although the sand off the track is different. The most notable difference is that there are tons of trees there, more than you'd ever see all at once in the final. The multi-coloured barriers don't seem to be there either, and the map is unlike any course in the game.
The second one resembles Broken Pier slightly, but the background, map, and tile set are entirely different. The sides of the pier have little poles there, but the final has blocks that are broken if you cash into them, leaving racers at danger to falling off the bridge. It's worth noting that although the same racers from the first picture are here, there's a fourth one behind Mario on the map.
The HUD is a lot simpler from the final. In the images, there's only the map, lap, item, and time. The final has those, plus the racers who in are the top four positions, coins, and your current position. The "LAP" and "TIME" text are different in the final, having a more block look, compared to the cartoony look of the early screenshots.
This old version also appears in an early video (YouTube link), which also reveals that the mugshots, items, and direction icons were all different. Interestingly, the position thing doesn't work, as it's just in the order of the Driver Select screen. Plus, it has all eight racers on it, but it's only four on it in the final!
Nintendo Official Magazine (October 2000, Issue 97, page 92) had a quick look at "Mario Kart Advance" during their preview of the GameCube and Game Boy Advance. Nothing they say is worth noting, other than how there are many details that fly past when you race, but is very unlike the final, which is almost bare of features.
And is it just me, or is the lingo they use in the magazine brain-shatteringly stupid?
For comparison, here's a screenshot from the final.
Different in every way, but for the better? I think not.
The following images are from Nintendo's US site, and appear to be from a version later in production, but still not finished.
This is Shy Guy Beach, and the map and area seem to be mildly different between the two, with the map smoothened slightly, and little extra things added in, like splits with water on the path, and such. The "Lap" part of the HUD is all the same size, but only the L is big in the final. The "Time" text also appears to be broader in the prerelease, and the mugshots seem slightly different. However, because of the stupid NP logo, I can't see if the position number is different.
And yes, I edited the player positions just to get the mugshots the same as the prerelease. =(
Boo Lake is showcased, and strangely, it uses the old bubble-font HUD. The mugshots are, once again, different, and Mario has passed the starting line; in the final, the "Time" is replaced with the amount of time you spent on the previous lap, but the image still has "Time" there. With no time.
Other differences include some altering of the map (mainly broadening some parts), the sky looking barely detailed, the level looked much brighter in general, and I can't tell if there are breakable blocks on the left side or far end.
More position editing by moi. =(
Once again, old HUD in action, with different mugshot action. However, other than the map being made a big more even in it's width, there's little else different I can see.
Same deal with the HUD and mugshots and whatnot. I'm kinda clueless with this one, as the level looks essentially the same the whole way through, so I'm unsure if I even got the right area or not. The only things I can tell are that the tepees aren't in that specific area in the final, I think.
And we're back to the more finalised HUD. The tiles are quite different for the grass and sand, the map is quite different (the prerelease varies in size at parts and is missing the little semi-oval behind the starting line), and that red shell icon may be different, but like super duper emphasis on may. Hence the bold.
This is of battle mode, but I've never tried multi-player, and therefore have no clue if anything is different, besides the mugshots.
[UPDATE] Now that No$gba has actually proved itself to not be a terrible competitor to VisualBoyAdvance by incorporating multi-player, I now know that litle has changed here, other than the change of background, addition of clouds, and more item boxes. And a different map and mugshots.
And this is the player select screen, but note how it's the 4-Player mode's player select. I've no idea if the multi-player player select is different from the single player one, so unless I get confirmation, I guess this'll just sit here.
[UPDATE] Now that I know what multi-player is like, it appears that the player select screen doesn't change; this is plain and simply an earlier version.