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BDSP deets
>no megas
>no z moves
>dynamax available for online play
>no artstyle change
>platinum dex available post game
>only a few lines of dialogue added
>all graphics and story is copied from dp
>battle tower copied from sword and shield, but with BDSP graphics and characters
>many gen 5+ pokemon available post game
The only non obvious thing I can tell you is mega's are going to be officially retconned. They will do this via Canalave library texts talking about alternate universes.