So checking out the creator interview from famitsu that came out last week, it's pretty interesting. They definitely wanted to try something new with the game and to that end they pretty much worked the game from the story and stuff from the ground up. There'll be similarities but a lot of it will be new. Even the world map isn't the same this time around.
On the characters, they apparently added a lot more for Maxim and Selan to work use as backbone. I think this is because their relationship is such a key factor in the end of the game so I guess they wanted to flesh it out a bit more. Tia is pretty different now, personality and image-wise she's a little different and it seems like they make take her story a bit further than it was last time.
On Guy's design, the old version was the strongman but a thin build. So they thought it would be a nice change to make him a lot more fitting of the strongman role this time around (hence his current design.
Artea is largely the same in concept, his design seems to be the only major difference (purple-haired dainty elf vs Blonde pretty boy elf, eh same thing).
On Dekar, because he has a pretty important scene in comforting Tia, and I think they expanded it a bit more and may have Dekar jump into the triangle for some of it (probably silliness). They actually helped reflect his master of a thousand techniques gag a bit in his gameplay elements. He's able to use all the weapons of the other characters apparently.