I think it is quite obviously going to be a fresh reboot with some transferable content. They wouldn't announce a niche "graphical" update to an already launched 8 year old game (thats had a horrible NA reception overall) on an XBOX live event guys.
There will likely be some character/mtx progression but not too much, else it will just alienate new players and re-doing all old content is not going to happen.
If they were going to re-design all the older content, then trust me they would have just waited until 2021 when genesis is out before releasing the game to NA so that it got the best reception, but they didn't do that. This indicates that the old content is going to be left as is and very separated from the new content, and also very obsolete (if it's even accessible at all).
As per Sega norms, they are keeping shut about it for now because they know people won't want to spend money if they think its all going to be meaningless in the long run.
Catch up at launch is going to be way too offputting with all the released content, so it would be just wasted to have this big an update just for a handful of hardcore fans.