In case it wasn't clear, "Champion Mode," which is the role-playing career of fictional fighter Andre Bishop, is a wholly separate mode in Fight Night Champion. Legacy, the old career, will still return, with improvements.
The big deal seems to be the addition of an experience point system replacing the old training structure whereby a minigame boosted only a specific set of attributes. Now it sounds like everything you acquire - both in training and the ring - goes into a pool, which you may then allocate for attribute boosts and skill perks down the line. For those who had hangups in one or more of Round Four's training games, this is a much more equitable way to balance yourself out as a fighter.
Out-of-the-ring stuff like fight promotion and sponsorships will also be incorporated, and the globetrotting gym training schedule from EA Sports MMA seems to have an analogue here, as well. On the whole, these look like upgrades to what I've always considered this franchise's true strength: an immersive and long-lasting career.