There are actually some pretty big differences. Characters cut, weapon functions altered, no hazard course, changes to levels. The basic plot and map progression will stay the same, but this isnt just a port from the old engine with some replaced AV assets, this is a recreation from the ground up.
I knew they were changing out everything in regards to graphics/animations/voices, but all the levels/gameplay/screenshots I've seen have looked very close to the original.
Maybe there's new footage out there I haven't seen, the last batch of footage I saw was like 3-4 years ago.