DSR added a lot of new things actually.
For starters, Issac is no longer a mute protagonist.
the combat got a slight overhaul to be streamlined and smoother like DS2 and 3.
The anti-gravity sections were completely redone to be zero G section just like DS2 (in the OG you had to pick a spot to 'jump' to in the zero g areas)
They changed a couple characters around to better fit the story.
They also mention a lot more characters and references to existing dead space media like dead space downfall and extraction.
And my personal favorite: you can backtrack now. In the og you were stuck with wherever the train had you at, now that the game was streamlined to be one massive map, you can go back to a previous area if you forgot a collectable or something so the game is less linear.
DSR was a love letter to dead space fans.