The great thing about MegaTen is the sense of familiarity I get every time I play a new game in the series (spin-off or whatever, it doesn't matter).
It's what makes you want to level up faster so that you can fuse Jack Frost as soon as possible, it's what makes you want to give him Agi and Null Fire, it's what makes you think "shit is getting real" when you see one of the Big 4 Angels before you can even read their names, it's what makes you feel like a confident veteran when you cast Zio on a Nekomata, it's what brings you back down to earth bruised and bloody when you realize they changed some of the weaknesses.
This won't happen with a complete redesign, and it would suck.
What would be great, though, is changing little bits to make the demons feel "fresh". For example I loved how badass they made Raja Naga in Raidou.
This is great:
The SMTIV examples are not.