It is of course up to each individual watcher what level of change is too much. To me:Star Trek doesn't take place in any world. It takes place in our imaginations. And that means it can be freely reimagined
Visuals
Technology
Continuity
Characterisation
Tone/attitudes
All being altered at the same time as they have on Discovery, means it's as much TOS' universe as Smallville is Superboy's, regardless of CBS' party line.
Without the same visuals, tech, attitudes, continuity and characterisations, TOS is only the vaguest outline in the future of Star Trek: Discovery, much like the Christopher Reeve Superman movies are the future both Smallville and Superboy are very vaguely leading towards.