My assumption is that the "new" animatronics essentially use parts cannibalized from the original ones, and that there might be two versions of them now: One that's more shiny, new, and closer to what they looked like in their prime, while the old ones have been left to decay even further, having been more or less replaced by the "new" ones.
The original Bonnie being faceless and limp calls into question if their function has changed. Maybe they'll behave akin to Golden Freddy, or perhaps even like Foxy, attacking the player character based on the amount of noise they make rather than the frequency of checking up on them via camera. This is pure speculation, however.