This new iteration of the Ecto-1 is more accurate than the actually prop they used in the film. That car has been inaccurate ever since it was left to rot, had parts pulled off and then got restored with several inaccuracies. This new car, as pointed out, is more faithful to the original look in '84.
They brought back the proper front lightbar bracket, properly painted the equipment rack black and the red and white antennae dome standard white, they have the red-poled tv antennae pointing the right way, the passenger side tube doesn't look green any more, the big red lens has been put back on the rotating light next to the siren and above the front lightbar...
The only noticeable changes to me are the terminus of the blue hoses, a radiological sticker on the tv antennae array, the slightly different proportions of the said array (although it does remind me of the Ectomobile line drawing used in such products as "Ghostbusters Meet the Laser Ghost), the checkered yellow and black tape is missing from the directional dome antennae (which could have just weathered off with time), and the ladder moved to the other side. And we now know why the ladder is on the other side, but it was forgivable before because the suicide doors on the passenger side made sense for simply framing a shot.
Besides all that... it would make sense if modifications would be made to a vehicle in service for a few years. things would change. Parts might get upgraded or swapped out. Additions get tacked on or configurations change for ease of use or even visibility (having a ladder in blocking your rear passenger view would suck).
Personally, I think the prop people did a fantastic job, and I'm very excited about the new functionality the car has with that new rear door equipment.