Not true at all. The models were redone. Hell, Johnson's model matches the actor the used in the CGI cutscenes. The Marines armor is very different as well. Textures too. HALO 2A uses some design philosophy from later games, but they are not reused. Even the elites are a hybrid design wise. Definitely look different.They just touched up a little here and there on the already existing assets from Halo Reach and ported them over. That was the benefit of that particular game series assets being so largely self-similar. It makes Halo is an exception to how remakes are handled. You shouldn't you this one example alone to show how Microsoft handles their remaster better than Sony.
What they did use was the animation and rigs from H2 for purposes of matching the original for switching