This is trivially obvious. And yet I have had people -- otherwise quite sensible, intelligent people -- seriously argue that if D&D magic worked it would NOT have changed the middle ages appreciably -- because people already believed it worked, and therefore they'd still behave the same.

If you assume magic worked all along, nothing we know in history would be the same.

Basically, at whatever point you assume useful, reliable magic enters the world, at that point the world is changed.

Exactly HOW it's changed is determined by what you allow magic to do and what limitations -- tactical, strategic, etc. -- it has.