I say go for it, if you dare. I mean, sure it is suposta be the same city, but in making it top-down, they had to change much of it. It is going to require a lot of work.
1: Making the main characters. You have about 8 people to make/remake. People like 8-ball are already there, but his hands aren't messed up for instance.
2: Scripting the missions. Shouldn't be too difficult for a good scripter. I didn't see too much in Advance that you couldn't do in III. Well, maybe the football thing...
3: Submissions. They work a little differently in Advance vs. III. You may need to base this on the Liberty mod on Vice.
4: Changing up the map. Even with the inconsistancies of the map, you could still use III's Liberty...Actually, make that LCS's Liberty. You would need to have Toni's hideout still there until mid storyline and the redlight district/hepburn hights would need to be like III's. And that's just part of Portland.
5: Placement of hidden objects. Lets face it, the "hidden packages" were not really hidden.
This is really just the basics. I do urge someone to pick this up, but I'm not holding my breath.